# data[].attributes.attachment_link.description
The history of life on Earth began about 3.8 billion years ago, initially with single-celled prokaryotic cells, such as bacteria. Multicellular life evolved over a billion years later and it's only in the last 570 million years that the kind of life forms we are familiar with began to evolve, starting with arthropods, followed by fish 530 million years ago (Ma), land plants 475Ma and forests 385Ma.

# data[].attributes.attachment_link.title
Saca la lengua - ¿Cómo funciona la secuenciación de las muestras? Entrevista a Núria Andreu

# data[].attributes.attachment_link.description
Nuria Andreu es bióloga, y trabaja en la Unidad de Genómica del CRG. Forman el equipo 7 técnicos más, una bioinformática, una lab manager y un jefe de grupo. La unidad ofrece servicio a los proyectos científicos sobre las tecnologías de microarrays (para el análisis de perfiles de expresión y variaciones estructurales a nivel genómico) [...]

# data[].attributes.body
A learning experience

# data[].attributes.attachment_link.title
Tiverton High School House Learning Experience - Reduce, Reuse and Recycle

# data[].attributes.attachment_link.description
TIVERTON HIGH SCHOOL students have pledged to cut down on the amount of waste sent to landfill sites, and to help increase recycling rates. The school's student led Eco-Club joined gifted and talented geographers in arranging an afternoon of learning on Friday 3 May.

# data[].attributes.attachment_link.entities[].name
Environmental Chater High School of Lawndale

# data[].attributes.attachment_link.entities[].name
Central Marine Fisheries Research Institute

# data[].attributes.attachment_link.title
"A Day in the LIfe" teaches how to reduce, reuse & recycle!

# data[].attributes.attachment_link.description
This award winning short film follows a family of four through their day commenting on the choices they make with cool facts and green solutions. Written by students at the Environmental Chater High School of Lawndale, directed by future super-star filmmakers Jordan Howard and Alex Pine.

# data[].attributes.attachment_link.title
BBC - GCSE Bitesize: Humans and their environment activity

# data[].attributes.attachment_link.description
A secondary school revision resource for Edexcel GCSE Science about evolution, humans and their environment

# data[].attributes.body
chrome-extension://oemmndcbldboiebfnladdacbdfmadadm/https://www.louisianabelieves.com/docs/default-source/academic-curriculum/task---social-studies---grade-1---impact-on-the-environment-pdf.pdf?sfvrsn=2

# data[].attributes.attachment_link.description
This versatile Science Making Connections design is an excellent wearable project choice for an authentic assessment of a student's knowledge of a particular cycle. It would be great for illustrating any numbers of cycles including: life cycles, food webs, rock cycle, metamorphosis, water cycle, photosynthesis, carbon cycle, and more.

# data[].attributes.attachment_link.title
How Life Began Origins Nova Neil Degrasse Tyson

# data[].attributes.attachment_link.title
The Fossil Record | Free Lesson Plans | Teachers

# data[].attributes.attachment_link.description
The Lesson Plan Library offers high school lesson plans covering all major school subjects and special interests.

# data[].attributes.body
worksheet

# data[].attributes.attachment_link.readable_text
Confronting Student Misconceptions
in a Large Class - by Dan Udovic

Small group evolution handout

Listed below are several answers to the following question, taken from student answers on a past exam. For each response, indicate whether you think an evolutionary biologist would agree with it, and if not, why not.

QUESTION: Fossil evidence suggests that the ancestor of the modern-day bat resembled a shrew or mouse and could not fly. How can the evolution of bat wings from the paws of shrew-like ancestors be explained?

ANSWERS:

1. Bats would be better adapted if they had wings, so gradually they developed them. Bats in each generation had better wings than their parents did.
PROBLEM: Lamarckian, goal-directed. Sounds like bats developed wings because they "wanted" or "needed" them (Lamarck's internal will (note: this is the instructor's annotation, it would not appear on the students' sheets).

2. Because the environment of the shrew-like ancestors favored gliding or flying, mutant individuals arose that were able to glide. Selection favored these individuals, and eventually all of them could glide. Repeating this process led ultimately to modern day bats.
PROBLEM: Changes are not induced by the environment. Mutations are independent of selection pressures. Does this mean that selection is "random"? NO! Otherwise, this answer is close to satisfactory. It does describe the process of natural selection (differential survival and reproduction), and does describe how cumulative selection could lead to the evolution of a complex trait.

3. The existence of structures as complex as a bat wing can not be explained by traditional evolutionary theory because structures like these are too complex to arise by chance.
PROBLEM: Assumption is that because there is no intelligent designer, that the process is solely "chance". Apply the same logic to motion of the planets. What is the probability that the Earth's motion, by chance, would form such a perfect and consistent ellipse around the sun? Ignores the power of cumulative selection.

4. Because the shrew-like ancestors of bats needed to fly to catch food, nature allowed them to develop the ability to fly better and they turned into bats.
PROBLEM: Need does not direct change. If it did, then wouldn't nature always allow change that prevents extinction. What does it mean to say "Nature allowed"?

5. The bat evolved wings in order to make it easier to catch its prey.
PROBLEM: Evolution is not goal-oriented. Who is "the bat", and is it capable of making conscious decisions about the future of the species? This statement just ignores the whole issue by not defining what "evolve" means. Is it like going to the grocery store to pick up some wings?

6. The shrew-like ancestors of bats kept stretching the skin and arms while jumping from tree branch to tree branch because that would help them glide better and jump farther. Gradually, through continued use of their arms in this way, they developed into wings.
PROBLEM: Lamarckian inheritance of acquired traits, use and disuse of parts. Discussion in class indicates this statement may just be too vague. From one point of view, a Darwinian might agree with it, from a different point of view, a Lamarckian might also.

Tell me more about this activity:

# data[].attributes.attachment_link.description
Confronting Student Misconceptions in a Large Class - by Dan UdovicSmall group evolution handout Listed below are several answers to the following question, taken from student answers on a past exam. For each response, indicate whether you think an evolutionary biologist would agree with it, and if not, why not.

# data[].attributes.attachment_link.title
Molecular Visualizations of DNA (2003) Drew Berry wehi.tv

# data[].attributes.body
for the video

# data[].attributes.attachment_link.readable_text
What is the evidence for evolution?

The theory of Biological Evolution makes two very bold claims about living creatures:

First: All living things on earth are related. They evolved from a common ancestor.

Second: The evolution of living things is powered by natural processes. Things which can be studied and understood.

But is there really any evidence that these two claims are true?

Yes. There are so many observable facts from so many different fields of study that the only way we can even begin to talk about them is to group them into categories or lines of evidence.

To keep things simple, here we'll focus on Evolution's first claim that: All living things on earth are related.

We cannot tackle the entire tree of life at once (after all there's an estimated 8.7 Million species alive today), so instead we'll focus most of our attention on one fairly small but fascinating branch of the evolutionary tree: Cetaceans. This branch includes whales, dolphins and porpoises.

Biologist claim that all these creatures are closely related, and that the entire group evolved from an ancient 4 legged land mammal.

Instead of taking their word for it, let's look at the facts. We'll start with a few from field of comparative anatomy: the study of differences and similarities between living things.

Whales live in water and from a distance, they sort of look like giant fish. A close inspection of their anatomy however, tells us a very different story.

Whales are like land mammals and unlike fish:

have placentas and give live birth

they feed milk to their young

they are warm blooded (which is extremely rare for a fish)

and whales do not have gills, instead - just like us - they breath air with two fully developed lungs.

Whales don't seem to have noses like mammals do. Instead they breathe through blowholes coming out the tops of their heads. Some whales have two blowholes that almost look like nostrils, but dolphins and porpoises only have one. Surprisingly, if you look at their skulls, you find that the blowhole splits into 2 nasal passages inside the head. Could it be that the blowhole is actually a highly modified mammal nose? It looks that way but we'll need more evidence to be sure.

Many whales have hair, just like land mammals. In this photograph, you can actually see the whiskers of this baby gray whale as he rests his chin on mama's back.

Strangely, whales have arm, wrist, hand, and finger bones inside their front flippers. Here's a photo of these bones, the same bones that bats, hippos and people have in their front appendages: One bone, two bones, wrist bones and finger bones.

Modern whales do not have back legs but they do have a pair of strange tiny bones where the hips and hind legs should be. Here's a picture of these bones from a bowhead whale. They almost look like shriveled hip, thigh, and shin bones. This one even has what looks like a deformed ball and socket joint between the hip and thigh bone, just like the ball and socket joint in your own hip. Is this resemblance a mere coincidence or are these real leg bones? Perhaps leftovers from the whales evolutionary history?

Before we draw any bold conclusions, let's see if a completely separate line of evidence will confirm our suspicions.

Embryology is the study of how creatures develop before being born or hatching from an egg.

Here we see a dolphin and a human embryo, side by side, at similar stages of development. Notice that they both have what look like arm buds, and leg buds. In humans, the leg buds grow to become legs. In whales, they grow for a while, but then stop, effectively fading away as the rest of the whale continues to grow.

These are all photographs of a common dolphin at different stages of development. Notice that early on, we see two nostril grooves on the front of the face, just like you'd expect in a puppy or a human.

As the dolphin continues to grow, the nostril groves migrate to the top of the head and fuse together becoming the dolphin's blowhole.

So far we have multiple facts from two independent lines of evidence, comparative anatomy, and embryology, both telling us the exact same story: The ancestors of whales were once 4 legged land creatures! Will the fossil record act as a third witness confirming this idea?

These are two species of extinct basilosaurid whales!

These creatures are known from multiple well preserved skeletons. They appear to have lived side by side roughly 34 to 40 million years ago.

In this photo we are looking down at the top of a basilosaurid skull. This is not a model or a cast, these are the actual bones which were pulled from the ground. Notice that the nasal opening is not on the top of the head like those of modern whales, and not at the end of the snout like those of most land mammals. Instead their nostrils sit right in the middle, this is an intermediate species, exactly what the theory of evolution tells us we should find!

At the back-end of a basilosaurid's body, there are small, yet fully developed hips, legs, ankle, feet and we suspect they had at least 3 toes though we've only found the bones for one.

These legs are far too small for walking on land, but may have been useful for mating or scratching away parasites and itchy skin.

Evolutionary theory tells us that the further we go back in time, the harder it will be to distinguish whales from regular land mammals.

Meet Maiacetus. Scientists have found multiple well preserved skeletons of this species, one of which appears to be a pregnant mother.

The hip bones of Maiacetus do seem sturdy enough to walk on land, but this animal is considered to be a whale for many reasons:
Their skeletons have all been found among fossils of sea-creatures

Their short legs combined with long flat fingers and toes, suggest they were strong swimmers with webbed hands and feet.

Here we see the bottom side of a maiacetus jaw and skull as it looked at the dig site. Her teeth match those of the basilosaurid whales we saw earlier.

And unique structures of her middle ear bones, the bulbs behind her jaw, match those of basilosaurid whales and modern whales.

Maiacetus appears to be, a walking whale!

The fossils of many ancient whale-like mammals have been found, and people continue to find more. Together, these fossils blur the line between 4 legged land mammals and fully aquatic whales, solidifying the idea that whales indeed, evolved from land creatures.

Now lets look at a 4th line of evidence: DNA?

DNA molecules contain chemical codes which act like recipes for living things.

Without ever looking at bones, embryos, or anatomy, researchers can compare the DNA code of different living creatures to find out who is most closely related to who.

Whale DNA has been compared to all kinds of other animals: fish, sea lions, you name it, and so far, the closest genetic match, is to the pudgy, water-loving hippopotamus.

This does not mean that whales evolved from hippos, but if this genetic finding is correct, whales and hippos both evolved from a common ancestor which lived roughly 54 million years ago.

At first the link between whales and hippos surprised researchers. Whales are mainly carnivores - they eat things like fish and small crustaceans, while hippos are mostly vegetarian.

A closer look however, reveals that hippos and whales, actually share many strange features, some of which may have come from their common ancestor.

Ancient walking whales have specially shaped ankle bones, found only in hippos and the close relatives of hippos, hippos, just like whales, often give birth and even nurse their young underwater, they both have multi chambered stomachs (which is common for herbivores but unheard of in fish-eating mammals), they are both missing a coat of fur, and here's a fun fact - whales and hippos are some of the only mammals on earth that have internal testicles.

So there you have it, dozens of facts from 4 independent lines of evidence, all tell us the exact same story, whales evolved from 4 legged land mammals, but the history of whales isn't the only evolutionary history that we've been able to work out.

We know from fossils, DNA, embryology and many other lines of evidence that bird wings are actually modified arms and claws! Birds evolved from dinosaur-like ancestors.

We can also clearly see that bat wings evolved from 5 fingered hands, similar to those of monkeys and shrews.

We've found that humans share a fairly recent common ancestor with chimpanzees, that mammals evolved from reptile-like creatures, those reptile-like creatures evolved from amphibian-like creatures, those amphibian-like creatures evolved from fish-like creatures, and fish if you go back far enough, share a common ancestor with segmented worms.

So to sum things up, thousands of observable facts from completely independent fields of study, are coming together to tell us the exact same story.

All living things on earth are related.

I'm Jon Perry and that's a basic overview of the evidence for evolution, Stated Clearly.

# data[].attributes.attachment_link.title
What Is The Evidence For Evolution? * Stated Clearly

# data[].attributes.attachment_link.description
What is the evidence for evolution? The theory of Biological Evolution makes two very bold claims about living creatures: First: All living things on earth are related. They evolved from a common ancestor. Second: The evolution of living things is powered by natural processes. Things which can be studied and understood.

# data[].attributes.attachment_link.readable_text
The Clock of Eras is a graphic aid to help us visualize geologic time. It is nearly impossible for the human mind to comprehend the amount of time that it has taken for the Earth to develop to its present state, yet we try to imagine each stage of its unfolding and the time that passed during each phase of development. The Clock of Eras uses the analogy of a circular clock to represent the development of our planet in geologic time. One can see at a glance the relative time lengths of each major geologic era. So how does this Clock work? The Clock represents geologic time on the Earth since its birth to the present, from the initial events that brought about the formation up to now. Each hour represents approximately 375 million years.

We can't take credit for the idea of this Clock. It is a concept that was developed in Montessori Education. The colors as used in the Montessori clock relate to the location of the life that was present during the time. So the Paleozoic Era is blue because life was primarily in the seas, the Mesozoic is brown because life moved to the land, and the Cenozoic is green because of the fresh new life: the mammals.

The Clock of Eras has been modified several times already and will continue to change over time as scientists learn more and more information through their research and discovery.

Click on the links below the clock for a description of each time period.

The first period of the clock of eras is the Hadean

A description of the Archean

A description of the Proterozoic Era

A description of the Paleozoic Era

A description of the Mesozoic Era

A description of the Cenozoic Era.

A clock of eras chart. Click here for a printable version.

A blank or non-colored chart Click here for a printable version

# data[].attributes.attachment_link.title
The Clock Of Eras One way to Visualize Geologic Time

# data[].attributes.attachment_link.description
The Clock of Eras is a graphic aid to help us visualize geologic time. It is nearly impossible for the human mind to comprehend the amount of time that it has taken for the Earth to develop to its present state, yet we try to imagine each stage of its unfolding and the time that passed during each phase of development.

# data[].attributes.body
Geologic History

# data[].attributes.attachment_link.entities[].name
Fundación Española para la Ciencia y la Tecnología

# data[].attributes.attachment_link.entities[].name
del Centro de Biología Molecular Severo Ochoa

# data[].attributes.attachment_link.authors[].name
CENTRO DE BIOLOGÍA MOLECULAR SEVERO OCHOA (CBMSO)

# data[].attributes.attachment_link.description
Nuestras capacidades como seres vivos dependen de las células. ¿Cuáles son sus principales componentes? ¿Por qué las dotan de características tan únicas en la naturaleza? Dentro de la célula/Inside the cell es serie de microvídeos del Centro de Biología Molecular Severo Ochoa que dan respuesta de una manera sencilla a ocho grandes cuestiones de la biología molecular de la célula.

# data[].attributes.attachment_link.title
Scientific Poster Design - Good and Bad Examples! (Poster Tutorial Part 2)

# data[].attributes.body
Tips

# data[].attributes.attachment_link.title
Scientific Poster Design - Good and Bad Examples! (Poster Tutorial Part 2)

# data[].attributes.attachment_link.description
Film biography of Gregor Mendel (1822-1884), the Moravian monk who became the father of modern genetics through his study of pea plants. One of a series of medical history films by J. Lee Sedwick, M.D., F.A.C.S., Clinical Professor of Surgery, East Carolina University, and Larry Gardner, President of Digifonics, Inc.

# data[].attributes.attachment_link.title
(OLD VIDEO) Chromosome Numbers During Division: Demystified!

# data[].attributes.attachment_link.title
Mitosis vs. Meiosis from Thinkwell's Video Biology Course

# data[].attributes.attachment_link.title
Mitosis: The Amazing Cell Process that Uses Division to Multiply! (Updated)

# data[].attributes.attachment_link.title
mitosis 3d animation | Phases of mitosis | cell cycle and cell division | mitosis and meiosis

# data[].attributes.attachment_link.title
A Science Odyssey: You Try It: DNA Workshop

# data[].attributes.attachment_link.entities[].name
Scienctific Practice and Effects of Money

# data[].attributes.attachment_link.title
How we think complex cells evolved - Adam Jacobson

# data[].attributes.attachment_link.title
The wacky history of cell theory - Lauren Royal-Woods

# data[].attributes.attachment_link.title
(OLD VIDEO) Cellular Respiration and the Mighty Mitochondria

# data[].attributes.attachment_link.description
One of the best in the country for Education, Training for ISEET, CET, NEET, NTSE, etc. Bags several ranks every year and their students get admission to prestigious engineering institutes such as IIT's, NIT's, etc. in large numbers. Run by professionals with a vision. Visit: www.ace-online.co.in

# data[].attributes.attachment_link.entities[].name
Arrival HD BBC Documentary Animals & Nature

# data[].attributes.attachment_link.title
BBC The Cell  The Hidden Kingdom - Documentary

# data[].attributes.attachment_link.description
BBC The Cell The Hidden Kingdom - Documentary The first part explores how centuries of scientific and religious dogma were overturned by the earliest discoveries of the existence of cells, and how scientists came to realise that there was, literally, more to life than meets the eye.

# data[].attributes.attachment_link.description
What is Metabolism? What is Anabolism? What is Catabolism?

# data[].attributes.attachment_link.title
Diffusion, Facilitated Diffusion & Active Transport: Movement across the Cell Membrane

# data[].attributes.attachment_link.title
Nucleus, Nucleolus, and Nuclear Envelope

# data[].attributes.attachment_link.description
Tried exporting at least 20 different ways. Last ditch effort if this makes it. I don't like macs anymore Mr. Fyfe.

# data[].attributes.attachment_link.description
We can think of chromatin as the packaging material in which we keep our DNA. Chromatin is a structure made up of DNA, RNA and proteins and it's regulation effects how our cells express their genes. In this tutorial we will talk about it's basic structure, components and functions.

# data[].attributes.attachment_link.description
Within our nucleus, we house a region referred to as the nucleolus. In this tutorial we will discuss how the nucleolus is arranged and the functions it serves in regard to ribosome construction.

# data[].attributes.attachment_link.description
The easiest way to think of our nucleus is as the brain of our cell. It contains our genetic material and also regulates the construction of new proteins through the creation of mRNA's in transcription. In this tutorial we will briefly discuss the structure and primary function of the nucleus.

# data[].attributes.attachment_link.readable_text
1. The inner core is in the centre and is the hottest part of the Earth. It is solid and made up of iron and nickel with temperatures of up to 5,500°C.

2. The outer core is the layer surrounding the inner core. It is a liquid layer, also made up of iron and nickel.

3. is the thickest section of the Earth at approximately 2,900 km. The mantle is made up of semi-molten rock called magma.

The theory of plate tectonics

is the outer layer of the Earth. It is a thin layer between 0 - 60 km thick. The crust is the solid rock layer upon which we live. It is either or The earth's crust is broken into plates. Heat rising and falling inside the mantle creates generated by radioactive decay in the core. The convection currents move the plates. Where convection currents diverge near the Earth's crust, plates move apart.

Where convection currents converge, plates move towards each other. The movement of the plates, and the activity inside the Earth, is called the theory of plate tectonics .

# data[].attributes.attachment_link.title
BBC Bitesize - GCSE Geography - Plate margins - AQA - Revision 1

# data[].attributes.attachment_link.description
The inner core is in the centre and is the hottest part of the Earth. It is solid and made up of iron and nickel with temperatures of up to 5,500°C. The outer core is the layer surrounding the inner core. It is a liquid layer, also made up of iron and nickel.

# data[].attributes.body
Playing God

# data[].attributes.attachment_link.readable_text
Contribuir a desarrollar tu creatividad. Nosotros ya hemos empezado a desarrollar la nuestra tanto creando esta actividad como creando especies, mira nuestro "Reptavífero":

Motivarte para investigar la Evolución de una manera diferente.

Analizar nuestras concepciones sobre el mundo, nuestras formas de verlo y entenderlo.

Estudiar la "Historia de la Ciencia" y reconocer el trabajo de algunos grandes científicos.

Comprender la importancia de las pruebas y teorías científicas sobre la Evolución.

Contribuir a un rechazo rotundo del "Creacionismo de las especies biológicas".
¡PREPÁRATE PARA CONVERTIRTE EN UN DIOS!
Sí, vamos a "jugar" a ser los creadores de una nueva especie que, por medio de un Diseño Inteligente, podamos encajar perfectamente en la "Clasificación de los seres vivos" según el saber científico actual. Nada, sólo para engañar al resto de simples mortales.

Para ir abriendo boca nos interesa que visites el siguiente enlace de Internet:

"Henders", de Warren Fahy: Es una novela de ciencia-ficción muy interesante relacionada con la evolución de las especies. Te la recomendamos. Si quieres saber más, visita el siguiente enlace: http://fernandovazquez.es/blog/?p=1210

También te recomendamos que veas los tres capítulos basados en la novela "Henders":

# data[].attributes.attachment_link.description
Contribuir a desarrollar tu creatividad. Nosotros ya hemos empezado a desarrollar la nuestra tanto creando esta actividad como creando especies, mira nuestro "Reptavífero": Motivarte para investigar la Evolución de una manera diferente. Analizar nuestras concepciones sobre el mundo, nuestras formas de verlo y entenderlo.

# data[].attributes.body
Esta en español, pero es que está tan bien explicado...

# data[].attributes.attachment_link.readable_text
El material genético se compacta en un área discreta de la célula formando los cromosomas. Éstos se encuentran en los virus, células procariotas, en el núcleo de células eucariortas y en cloroplastos y mitocondrias.

Diferencias entre eucariotas y procariotas

La diferencia fundamental está en la cantidad de ADN que es inferior en procaioras que en procariotas como por
ejemplo: en E. coli el ADN mide 1.3 mm y tiene 4.2 Mb mientras que una célula humana tiene 1.8 mm y 6000 Mb pero
si hay 10 elevado a 13 células, el ADN humano mide 2 x 10E13 m.

En la siguiente tabla se muestran las diferencias más importantes:

El cromosoma eucariótico

Los cromosomas se encuentran en el núcleo celular separados del resto de la célula por la membrana nuclear. Un cromosoma tiene tres partes fundamentales: centrómero, telómero y los brazos.

El centrómero (constricción cromosómica primaria) es la estructura a la que se une el huso acromático. La región centromérica aparece normalmente como un y su posición define la relación entre las longitudes de los dos brazos centroméricos que es una característica muy útil. Según la posición del centrómero los cromosomas se clasifican en:

Telocéntricos con el centrómero en un extremo.

Acrocéntricos con el centrómero alejado del centro.

Metacéntricos con el cromosoma en el centro.

Los telómeros son secuencias situadas en los extremos de los cromosomas que les dan estabilidad. Se encuentran sobre todo en los cromosomas eucarióticos lineales.

El número de nucleolos difiere de un organismo a otro, variando entre uno y muchos. Los nucleolos contienen ARN ribosómico, un componente muy importante de los ribosomas. Los nucleolos se encuentran situados en las constricciones secundarias de los cromosomas llamadas organizadores nucleolares, que ocupan lugares específicos en el cromosoma.

En los centrómeros y telómeros se encuentra asociado ADN satélite que son segmentos de ADN altamente repetitivo y moderadamente repetitivo.

Los cromosomas eucarióticos están la mayor parte del ciclo celular como una sola cromátida y como dos cuando se replica. La replicación del ADN es semiconservativa, esto se demostró en un experimento en el que se marcó con tritio una cromátida. Entonces se procedió a replicar esta cromátida en presencia de tritio y se obtuvo un cromosoma de dos cromátidas marcadas. Se hizo volver a replicarse, esta vez sin presencia de tritio y se obtuvieron cuatro cromátidas formando dos cromosomas. Cada cromosoma tenía una molécula marcada y la otra no con lo que en la replicación se conservaba para el nuevo cromosoma una de las cromátidas parentales.

Organización del cromosoma eucariótico

En células eucarióticas que no se hayan sometidas a división celular el cromosoma re- cibe el nombre de cromatina. La cromatina consiste en fibras que contienen proteínas, ADN (en cantidades muy parecidas) y una pequeña porción de ARN. Las proteínas que se asocian al ADN son básicas y se llaman histonas. Las histonas que participan son H1, H2A, H2B, H3 y H4 y su porcentaje de aminoácidos básicos y características son:

A continuación se desglosan por orden de empaquetamiento los diferentes niveles, desde el primero hasta el último sucesivamente:

Primer nivel: nucleosoma

Esta estructura vista al microscopio se ve como si fuera un collar de perlas del que las cuentas son los nucleosomas.

El nucleosoma está formado por un octámero de histonas en el que hay dos subunidades de las histonas H2A, H2B, H3 y H4 de las que sobresale un extremo amino de aproximadamente 30 aminoácidos que permite la unión de otras proteínas para las funciones de transcripción, replicación y reparación. Alrededor de este octámero se arrolla el ADN con dos vueltas (166 pares de bases aprox). El espaciamento entre las cuentas está formado por ADN que se llama ADN puente o ADN de unión.

El nucleosoma mide 6 nm y se considera la unidad básica de empaquetamiento del ADN. Los nucleosomas se vuelven a organizar con la ayuda de la histona H1 habiendo una por cada nucleosoma.

Segundo nivel: fibra de 30 nm.

El nucleosoma que contiene H1 se pliega en una conformación que se supone en zigzag (pero también se piensa que en hélice o con interacciones cruzadas) colocándose próximas a la salida y entrada del nucleosoma; y cuya apariencia sugiere que los nucleosomas interaccionan mediante contactos entre sus moléculas H1. Esta fibra que se forma tiene 30 nm de espesor, en el que se aprecian los nucleosomas. Las histonas H1 se disponen de manera que forman el eje central sobre el que se arrollan los nucleosomas. Por cada vuelta de la espiral que forma esta fibra hay seis nucleosomas. A este arrollamiento de los cromosomas sobre sí mismos se le llama solenoide.

Tercer nivel: fibra de 300 nm.

Algunos autores proponen que el solenoide a su vez se enrolla en bucles de 300 nm, formando otras estructuras llamadas cronómeros.

Si eliminamos las histonas del cromosoma en metafase mitótica se puede ver que los cromosomas tienen un esqueleto central densamente teñido. Desde este esqueleto se proyectan lazos de ADN que comienzan y acaban en el
esqueleto. Este esqueleto central llamado andamio está compuesto por la enzima toposiomerasa II (también llamada condensina que enlazan o desenlazan nudos o lazos en una cadena) y por proteínas SMC (Structural Maintenance of Chromosomes o Mantenimiento de la estructura del cromosoma) en el que parece haber regiones especiales llamadas regiones de unión al andamio. Para eso, se estima que existen las llamadas zonas SAR ( caffold Attachment Region o Regiones de Unión al Andamio) que son zonas de la fibra de 30nm (asas) muy ricas en amina y timina para unirse al andamio, dando lugar a una fibra de 300 nm.

El empaquetamiento en este punto es de 2.000 veces la hebra de ADN.

Cuarto nivel: cromosoma

Se produce por el arrollamiento de la fibra de 300 nm sobre sí misma formando una espiral y empaquetando el ADN hasta 10.000 veces formando una cromátide. La unión de dos cromátidas hermanas a través de las condensinas.

Cromosomas politénicos y plumosos

Los cromosomas politénicos o gigantes (miden más o menos 2 mm) se descubrieron en Drosophila y se han observado en otros dípteros. Estos cromosomas tienen bandas y se producen en ciertas células secretoras que no se dividen. Este sistema de bandas está muy especializado.

En estas célula,s Balbiani descubrió estrcuturas largas y con forma de salchicha que presentaban engrosamientos y estrías cruzadas. Esto eran cromosomas que se daban en los tejidos secretores de los dípteros aunque él no se dio cuenta.

Estos cromosomas eran cromosomas que se replican varias veces sin que se produzca una separación real de las cromátidas con lo que el cromosoma se alarga y se hace más grueso. El conjunto de réplicas se denomina cromosoma politénico. Este cromosoma queda unido por el cromocentro, que es la zona de fusión de las regiones heterocromatínicas situadas alrededor de los centrómeros de los pares cromosómicos. A lo largo del cromosoma politénico se encuentran estrías llamadas bandas que varían en anchura y morfología. Hay regiones engrosadas llamadas PUFFS y a veces muy distendidas llamadas ANILLOS DE BALBIANI que se piensa que corresponden a regiones de síntesis de ARN. No hay una relación uno a uno de bandas y genes, aunque se piensa que los genes activos están en las bandas más claras.

Los cromosomas plumosos son cromosomas cuya fibra de ADN tiene muchos lazos y su longitud varía entre 0.4 y 0.8 mm.

Es el conjunto de cromosomas y su morfología en metafase de una célula que se ordenan colocándolos por parejas de cromosomas homólogos.

Para los seres humanos hay 23 pares de cromosomas homólogos de los que 22 son autosomas (son los cromosomas no sexuales) y uno son los sexuales. En total hay 46 cromosomas de los que la mitad se heredan del padre y la otra mitad de la madre con lo que la especie humana es diploide (parejas de cromosomas homólogos).

Los cromosomas se agrupan en categorías (A-G, X, Y) según su longitud (están dispuestos desde mayor longitud hasta menor longitud), según el índice centromérico, según la posición de las constricciones cromosómicas y según la posición de las bandas de tinción. Los gametos son células n, es decir, con un sólo cromosoma: sin parejas. Así cuando se produce la fecundación, se producen células 2n. En el ciclo celular puede variar la importancia de la fase n o 2n según el tipo de organismo. El ser n o 2n no aporta la suficiente información sino que también es importante la longitud, I.C.,...

Heterocromatina y eucromatina

Si hacemos reaccionar el ADN con el tinte Feulgen hay una parte que se tiñe intensamente y otra que no tanto. La parte que se tiñe intensamente se llama heterocromatina y la parte que no eucromatina. La mayoría de los genes activos se encuentran en la eucromatina

La heterocromatina puede ser constitutiva o facultativa. La de tipo constitutivo es un rasgo permanente de una posición concreta del cromosoma y es un carácter hereditario. La heterocromatina facultativa puede estar presente o ausente en una posición determinada del cromosoma.

Fuente de la imagen: http://webs.uvigo.es/mmegias/5-celulas/ampliaciones/8-cromosomas.php

# data[].attributes.attachment_link.title
La organización del material genético (ADN) en cromosomas eucariotas | Ciencia y Biología

# data[].attributes.attachment_link.description
El material genético se compacta en un área discreta de la célula formando los cromosomas. Éstos se encuentran en los virus, células procariotas, en el núcleo de células eucariortas y en cloroplastos y mitocondrias. ÍndiceDiferencias entre eucariotas y procariotasEl cromosoma eucarióticoOrganización cromosoma eucarióticoPrimer nivel: nucleosomaSegundo nivel: fibra de 30 nm.Tercer nivel: fibra de 300 ...

# data[].attributes.headline
Laboratory experience. Genetic drift and evolution

# data[].attributes.subject
Laboratory experience. Genetic drift and evolution

# data[].attributes.body
Student worksheet

# data[].attributes.headline
Laboratory experience. Genetic drift and evolution

# data[].attributes.subject
Laboratory experience. Genetic drift and evolution

# data[].attributes.body
2008 Sinauer Associates Sadava, D. et al. Life: The Science of Biology, 8th ed. (Sunderland, MA: Sinauer Associates and W. H. Freeman & Company), 196–197.

# data[].attributes.attachment_link.title
Jack Szostak (Harvard/HHMI) Part 3: Non-enzymatic Copying of Nucleic Acid Templates

# data[].attributes.attachment_link.description
http://www.ibiology.org/ibioseminars/evolution-ecology/jack-szostak-part-3.html Szostak begins his lecture with examples of the extreme environments in which life exists on Earth. He postulates that given the large number of Earth-like planets orbiting Sun-like stars, and the ability of microbial life to exist in a wide range of environments, it is probable that an environment that could support life exists somewhere in our galaxy.

# data[].attributes.attachment_link.title
Jack Szostak (Harvard/HHMI) Part 2: Protocell Membranes

# data[].attributes.attachment_link.description
http://www.ibiology.org/ibioseminars/evolution-ecology/jack-szostak-part-2.html Szostak begins his lecture with examples of the extreme environments in which life exists on Earth. He postulates that given the large number of Earth-like planets orbiting Sun-like stars, and the ability of microbial life to exist in a wide range of environments, it is probable that an environment that could support life exists somewhere in our galaxy.

# data[].attributes.attachment_link.title
Jack Szostak (Harvard/HHMI) Part 1: The Origin of Cellular Life on Earth

# data[].attributes.attachment_link.description
http://www.ibiology.org/ibioseminars/evolution-ecology/jack-szostak-part-1.html Szostak begins his lecture with examples of the extreme environments in which life exists on Earth. He postulates that given the large number of earth-like planets orbiting sun-like stars, and the ability of microbial life to exist in a wide range of environments, it is probable that an environment that could support life exists somewhere in our galaxy.

# data[].attributes.attachment_link.title
Myths and misconceptions about evolution - Alex Gendler

# data[].attributes.body
para 4º y 1º Bach

# data[].attributes.attachment_link.title
(OLD VIDEO) Mutations: The Potential Power of a Small Change

# data[].attributes.body
Check this web site and read about Geological time periods. Make a presentation summarizing main events.

# data[].attributes.attachment_link.description
The history of life on Earth began about 3.8 billion years ago, initially with single-celled prokaryotic cells, such as bacteria. Multicellular life evolved over a billion years later and it's only in the last 570 million years that the kind of life forms we are familiar with began to evolve, starting with arthropods, followed by fish 530 million years ago (Ma), land plants 475Ma and forests 385Ma.

# data[].attributes.body
Unit 4