Name: how genome size shapes new genes
Uploaded: 2020-11-23T11:47:33.000Z
Duration: 3 min 21 s
Channel: JoVE

K12 Standards

north-carolina-k-12-science-standard-course-of-study

NC K-12 Science Standard Course of Study

9-12

Biology

Biological Evolution- Unity and Diversity

LS.Bio.9.2

genome-size-and-the-evolution-of-new-genes

Genome Size and the Evolution of New Genes

A pesar de la naturaleza simple del código genético, existe una variación considerable en términos de tamaño del genoma, desde los genomas más pequeños conocidos, incluida la proteobacteria Candidatus Carsonella ruddii con menos de 160 mil pares de bases, hasta los más grandes, como la planta con flores japonesa Paris japonica con alrededor ><de 150 mil millones.
 
A pesar de estos extremos, las bacterias y las arqueas generalmente tienen alrededor de 3.000 genes dentro de sus genomas. Debido a que los procariotas casi no tienen secuencias no codificantes, esto significa que sus genomas pueden ser relativamente pequeños en comparación con los de los eucariotas. Los genomas más pequeños también significan menos para replicar en cada ronda de división celular, lo que tiene sentido logístico para una reproducción rápida.
Los eucariotas suelen tener alrededor de 20.000 genes, pero sus genomas están puntuados por largos tramos de secuencia no codificante, lo que significa que el tamaño de su genoma no se traduce necesariamente en complejidad. 
El genoma de Paris japonica puede ser más de cincuenta veces el tamaño del genoma humano, pero esto se debe, al menos en parte, a grandes cantidades de secuencias no codificantes y probablemente a altos niveles de duplicación, no necesariamente genes más nuevos.
Entonces, ¿cómo evolucionan los organismos nuevos genes? La respuesta suele ser modificando la secuencia que ya existe.
Uno de los principales recursos para que los nuevos genes evolucionen es a través de la duplicación de genes. Imagina que una sección de ADN que contiene un gen se duplica accidentalmente. Ahora el organismo tiene una segunda copia de un gen existente. 
Estas nuevas copias de genes están libres de las restricciones impuestas al original para mantener su función, por lo que pueden divergir, lo que podría evolucionar hacia un papel novedoso o una función modificada del original. 
Otra forma de crear nuevos genes es la mezcla de ADN, en la que los segmentos de un gen existente o una copia de un gen se separan y se mueven para unirse a los de un gen diferente, creando un gen híbrido que puede asumir una nueva función.
Mutación intragénica: los cambios en una secuencia genética introducidos por mutaciones a lo largo del tiempo, explican muchos genes "nuevos". Esta divergencia es más notable cuando se comparan especies o linajes que a su vez divergen de forma independiente. Una vez que esta divergencia está más allá de cierto punto, o un gen adquiere una función nueva, pueden clasificarse como genes completamente diferentes.
Por último, la transferencia horizontal de genes aporta nuevos genes y secuencias al genoma procedentes de fuentes externas, como otros individuos e incluso otras especies. Este tipo de adquisición de genes nuevos es más común en procariotas y arqueas, siendo la transferencia de genes de resistencia a antibióticos un ejemplo bien conocido. 
Si bien es raro en los eucariotas, todavía se considera una fuente esencial de novedad genética, y el material genético puede provenir incluso de especies lejanas relacionadas, como las bacterias y los hongos en este ejemplo.

tamaño del genoma y evolución de nuevos genes

Si bien cada organismo vivo tiene un genoma de algún tipo (ya sea ARN o ADN), existe una variación considerable en el tamaño de estos modelos. Un factor importante que afecta el tamaño del genoma es si el organismo es procariota o eucariota.

En los procariotas, el genoma contiene poca o ninguna secuencia no codificante, de modo que los genes están agrupados estrechamente en grupos u operones secuencialmente a lo largo del cromosoma. Por el contrario, los genes de los eucariotas están marcados por largos tramos de secuencias no codificantes. En general, esto contribuye al fenómeno de que los genomas procarióticos tienden a ser más pequeños (es decir, contienen menos bases) en promedio que los de los eucariotas.

Como era de esperar, dada esta observación, los genomas más pequeños conocidos son, en su mayoría, procariotas. Candidatus Carsonella rudii, por ejemplo, es una proteobacteria muy simplificada cuyo genoma tiene un tamaño de sólo 160.000 pares de bases. Habiendo perdido muchos genes que necesitaba para sintetizar proteínas esenciales para la vida, ha evolucionado hasta convertirse en un simbionte intracelular obligado. En el extremo opuesto del espectro, la planta con flores japonesa eucariota Paris japonica es uno de los genomas más grandes conocidos, con alrededor de 150 mil millones de pares de bases. Aunque se desconoce la cantidad de genes que codifica, el genoma muestra grandes cantidades de duplicación y secuencias no codificantes.

Dentro del genoma de un procariota promedio hay, aproximadamente, 3.000 genes. El eucariota promedio tiene alrededor de 20.000. Pero el tamaño del genoma, especialmente en eucariotas, es tremendamente variable, en gran parte debido a la cantidad de secuencia no codificante.

La creación de nuevos genes

Para desarrollar nuevos genes, los organismos tienen algunas opciones principales. Lo único que la mayoría de estos tienen en común es que modifican secuencias que ya existen.

La duplicación juega un papel importante en la creación de nuevos genes y existen algunos tipos de duplicación que pueden dar como resultado estas nuevas secuencias. En la duplicación de genes, se duplica una sección de ADN que contiene un gen. Esta segunda copia no enfrenta la presión de selección que constriñe a la primera, por lo que puede divergir. Con el tiempo, esto puede conducir a la evolución de genes nuevos, con nuevas funciones.

Otro tipo de duplicación, la mezcla de ADN, puede dar lugar a que sólo una sección de un gen se duplique y se una a otro gen. Esto puede dar lugar a la creación de genes novedosos, con productos novedosos.

A veces, nuevos genes simplemente evolucionan a partir de mutaciones acumuladas a lo largo del tiempo. Esto se conoce como mutación intragénica y es más notable cuando se comparan especies o poblaciones divergentes.

Finalmente, también es posible obtener nuevos genes de fuentes externas, en un proceso conocido como transferencia horizontal de genes. Esto significa que se puede incorporar material genético de otros individuos, a veces de la misma especie, pero también potencialmente de otra especie completamente. Esta es una fuente frecuente de genes nuevos en procariotas y arqueas. Es menos común en eucariotas, pero se ha demostrado que ocurre, y los eucariotas pueden incluso recoger información genética de fuentes tan distantes como bacterias u hongos.

Tamaño del genoma y evolución de nuevos genes

Si bien todos los organismos vivos tienen un genoma de algún tipo (ya sea ARN o ADN), existe una variación considerable en los tamaños de estos planos. Uno de los principales ...

Education

Educación

JoVE Core

Molecular Biology

Biología Molecular

Unit 1

Unidad 1

DNA, Cells, and Evolution

ADN, células y evolución

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

how genome size shapes new genes

Genome size can vary widely, and it also helps show how new genes form. Every living organism has a genome, whether it uses RNA or DNA. Yet the size of that genome can be very different from one organism to another.

One major difference is whether the organism is prokaryotic or eukaryotic. Prokaryotic genomes usually have little to no non-coding sequence, which means genes are packed closely together in operons along the chromosome. Eukaryotic genomes contain long stretches of non-coding DNA between genes. This helps explain why prokaryotic genomes are often smaller than eukaryotic genomes.

The smallest known genomes are mostly prokaryotic. One example is Candidatus Carsonella rudii, a highly simplified proteobacterium with a genome of only 160 thousand base-pairs. It has lost many genes needed to make life-essential proteins, so it now lives as an obligate intracellular symbiont. On the other end of the range, the Japanese flowering plant Paris japonica has one of the largest known genomes, at about 150 billion base-pairs. Its genome contains extensive duplication and non-coding sequence, and the number of genes it encodes is not known.

Gene counts also differ between these groups. An average prokaryote has roughly 3,000 genes. An average eukaryote has around 20,000 genes. Even so, genome size in eukaryotes can vary a great deal because of how much non-coding DNA they contain.

New genes usually arise by changing sequences that already exist. Gene duplication copies a DNA segment that contains a gene. The extra copy is not under the same selection pressure as the original, so it can change over time and take on a new role. DNA shuffling can duplicate only part of a gene and join it to another gene, which can create a new gene product.

New genes can also appear through intragenic mutation, which means mutations build up within a gene over time. This is often seen when comparing different species or populations that have diverged. Another source is horizontal gene transfer, where genetic material comes from another individual or even another species. This is a common way for prokaryotes and archaea to gain novel genes, although it can also happen in eukaryotes, including gene transfers from bacteria or fungi.

How Genome Size Shapes New Genes

the dna helix

DNA Structure and Base Pairing

DNA has a helical structure that helps store genetic information. The double helix is the shape formed by two strands that twist around each other. This structure is a key feature of DNA in cells. The two strands are held together by base pairing. Bases on one strand match with bases on the other strand in a regular way. This pairing helps keep the DNA molecule stable and organized. The helix shape also helps explain how DNA fits into chromosomes and carries instructions for living things. By...

the central dogma

DNA to Protein: Central Dogma Steps

The central dogma explains how genetic information moves from DNA to RNA to protein. This flow is a key idea in molecular biology. It helps show how cells use genes to make the molecules that carry out work. DNA stores the instructions for making proteins. During transcription, a gene in DNA is copied into RNA. The RNA then carries that message to the next stage. In translation, the RNA message is used to build a protein. Proteins do much of the cell’s work, so this process connects genes to...

prokaryotic cells

Inside Prokaryotic Cell Structure

Prokaryotic cells have a simple internal structure compared with many other cells. They are usually divided into two groups: bacteria and archaea. These cells have no nucleus, so their DNA is not enclosed in a membrane-bound compartment. Instead, the genetic material is found in the nucleoid, a region where the DNA is located in the cell. Some prokaryotic cells also contain plasmids, which are small extra DNA molecules. A capsule may surround the cell and provide an additional outer layer. <br...

eukaryotic compartmentalization

Cell Compartments in Eukaryotes

Eukaryotic cells use compartments to organize many different jobs inside one cell. These compartments help separate processes so they can happen in the right place. The compartment called the cytosol is the fluid part of the cell. It surrounds the other internal structures and helps support many cell processes. Inside the cell, membrane-bound organelles create distinct microenvironments. A microenvironment is a small internal space with conditions that fit a specific task. This separation...

the tree of life - bacteria, archaea, eukaryotes

the-tree-of-life-bacteria-archaea-eukaryotes

How Life Is Classified Into Three Domains

The tree of life shows how organisms are related through evolution. Its root represents the common ancestor of all life on Earth. From that point, branches spread out like a tree. The tips of the branches represent living, or extant, species. Species that no longer exist, called extinct species, appear closer to the center of the tree. Today, these organisms are grouped into three domains: bacteria, archaea, and eukaryotes. Scientists still debate these divisions, so the tree of life keeps...

mutations

How Mutations Change DNA

Mutations are changes in DNA. They can alter the genetic code and affect how genes work. In high school biology, students often study mutations to understand how DNA changes can influence traits. The transcript focuses on two common mutation types: point mutations and frameshift mutations. A point mutation changes one nucleotide, which is one DNA building block. A frameshift mutation happens when nucleotides are added or removed and the reading frame shifts. These changes matter because they...

gene evolution - fast or slow?

Why Some Gene Regions Evolve Faster

Gene regions do not all change at the same rate. In eukaryotes, long stretches of DNA do not code for proteins or RNAs. Some of these non-coding regions include important regulatory sequences, but most have no known function. Because little or no selection pressure acts on them, they often change the fastest over evolutionary time. Protein-coding regions usually evolve more slowly. A change in a coding sequence can alter a protein and make it less effective at doing its job. That creates...

genome size and the evolution of new genes

Genome size can vary widely, and it also helps show how new genes form. Every living organism has a genome, whether it uses RNA or DNA. Yet the size of that genome can be very different from one organism to another. One major difference is whether the organism is prokaryotic or eukaryotic. Prokaryotic genomes usually have little to no non-coding sequence, which means genes are packed closely together in operons along the chromosome. Eukaryotic genomes contain long stretches of non-coding DNA...

gene families

How Gene Families Evolve and Diversify

Gene families are groups of genes that are thought to come from a shared ancestor. They usually form when a gene is copied by mistake during cell division. The original gene stays under selection pressure to keep its job, while the extra copy can change more quickly over time. Some copied genes gain a new role in the organism. When that happens, they are called paralogs, which are two genes in the same species that evolved from one common ancestral gene. Gene families also include orthologs,...

types of genetic transfer between organisms

types-of-genetic-transfer-between-organisms

How Genes Move Within and Across Generations

Genetic transfer is the movement of genetic information from one organism to another. It happens in two main ways: vertical gene transfer and horizontal gene transfer. Vertical gene transfer moves genes from one generation to the next. It is much more common than horizontal gene transfer. Vertical gene transfer happens during both sexual reproduction and asexual reproduction. In both cases, one or more organisms pass some or all of their genome to their offspring. This kind of transfer occurs...

key terms and concepts

Compare prokaryotic and eukaryotic genome sizes, then see how gene duplication, DNA shuffling, mutation, and horizontal gene transfer create new genes.

How Genome Size Shapes New Genes | JoVE K12

molecular biology

JoVE Core Biología Molecular presenta los procesos moleculares de la célula a través de lecciones claras y concisas de animaciones audiovisuales. Además, en los vídeos de científicos en acción, se muestran aplicaciones de estos conceptos mediante experimentos reales en laboratorios especializados.

new-jersey-student-learning-standards-for-science

New Jersey Student Learning Standards for Science

High School

Life Science

heredity-inheritance-and-variation-of-traits

Heredity: Inheritance and Variation of Traits

HS-LS3-1

Florida Department of Education

SC.912.L.15

SC.912.L.15.2

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What is Natural Selection?

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LS.Bio.9.3

Construct an explanation to explain how natural selection leads to adaptations within populations.

LS.Bio.9.4

Extinction

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LS.Bio.10.1

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Use models (including dichotomous keys, scientific nomenclature, cladograms, phylogenetic trees) to identify organisms and exemplify relationships.

LS.Bio.10.2

Understand natural selection as a mechanism for biological evolution.

How Genome Size Shapes New Genes

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