Name: how organelles pass genes to the nucleus
Uploaded: 2020-11-23T11:47:36.000Z
Duration: 2 min 19 s
Channel: JoVE

Organelle genomes, such as those in both mitochondria and chloroplasts, are smaller than those of their prokaryotic ancestors. This is because, during evolution, the majority of their genes were exported to the nucleus while many others were lost before developing into a mitochondrial or chloroplast genome.&#160;
These exported genes are known as nuclear integrants of organellar DNA.&#160; Specifically, the genes from the mitochondria are nuclear integrants of mitochondrial DNA, and those from the chloroplast are nuclear integrants of plastid DNA.&#160;
One theory of why cells may transfer the genes from mitochondria and chloroplasts to the nucleus is that the electron transfer reactions in mitochondria and chloroplasts generate mutation-causing free radicals. The export of these genes reduces exposure to free radicals and the likelihood of harmful mutations.
Additionally, the nucleus has a more effective DNA repair system than either mitochondria or chloroplasts.
As mitochondrial and chloroplast DNA are inherited from a single parent only, they cannot undergo sexual recombination. However, once the genes are incorporated into nuclear DNA, genes from both parents are inherited.&#160;
Sexual recombination allows rearrangement of genes from both parents which can prevent the accumulation of undesired mutations and can improve adaptation to the surrounding environment.&#160;
Nuclear DNA&#8217;s transcription and translation machinery are different from those of mitochondria and chloroplasts; therefore, the exported genes must undergo several modifications to function properly.&#160;
These changes include the insertion of new DNA sequences for a promoter and a terminator required for proper mRNA and protein production. A targeting sequence is also added to direct the protein product to the mitochondria or chloroplast.&#160;
Most exported genes retain their original function in the mitochondria and chloroplast.&#160; However, in some cases, it has led to the development of genes with new functions.&#160;

export of mitochondrial and chloroplast genes

Export of Mitochondrial and Chloroplast Genes

A eukaryotic cell can have up to three different types of genetic systems: nuclear, mitochondrial, and chloroplast. During evolution, organelles have exported many genes to the nucleus; this transfer is still ongoing in some plant species. Approximately 18% of the Arabidopsis thaliana nuclear genome is thought to be derived from the chloroplast&#8217;s cyanobacterial ancestor, and around 75% of the yeast genome derived from the mitochondria&#8217;s bacterial ancestor. This export has occurred irrespective of the location or the size of the gene in the organellar genome;&#160; large genes and, in some cases the entire organellar genome, have been found in the nucleus.
Gene transfer to the nucleus is coupled with the loss of the genetic autonomy of the organelle. However, many of the proteins coded by the exported genes are still produced by the nucleus and transported back to the organelle.&#160; This is possible as the genes are modified to be compatible with nuclear transcriptional and translational machinery and undergo changes such as the addition of a promoter and a terminator. A targeting sequence is also added, so the resulting proteins get delivered to the specific organelle. This also enables the nucleus to control the supply of these proteins and regulate the biogenesis of the organelles. Sometimes, such exported genes evolve and perform new functions for the organelles other than their parent one. For example, almost 50% of plastid-derived genes in Arabidopsis thaliana carry out non-plastid functions.
There are several theories as to why organisms transfer genes from the organelles to the nucleus. Both mitochondria and chloroplasts generate free radicals which can cause harmful mutations in their DNA. Transfering vulnerable organellar genes to the nucleus may be one of the strategies to protect them from mutations. According to the genetic principle Muller&#8217;s ratchet, asexual reproduction leads to the accumulations of deleterious mutations which eventually can cause the extinction of the species. However once transferred to the sexual genome of the nucleus, the exported gene can undergo sexual recombination which helps it to prevent the accumulation of harmful mutations. &#160;

A eukaryotic cell can have up to three different types of genetic systems: nuclear, mitochondrial, and chloroplast. During evolution, organelles have ...

export-of-mitochondrial-and-chloroplast-genes

Education

JoVE Core

Molecular Biology

Unit 1

DNA Replication

KEY TERMS AND CONCEPTS

how organelles pass genes to the nucleus

Eukaryotic cells can carry three genetic systems: nuclear, mitochondrial, and chloroplast. Over time, many genes from mitochondria and chloroplasts have moved into the nucleus. In some plant species, this gene transfer is still happening.

This movement has affected large parts of the genome in some organisms. About 18% of the Arabidopsis thaliana nuclear genome is thought to come from the chloroplast's cyanobacterial ancestor. In yeast, around 75% of the genome is derived from the mitochondria's bacterial ancestor.

Gene transfer can happen no matter where a gene sits in the organelle genome or how large it is. In some cases, even the entire organellar genome has been found in the nucleus. As these genes move, the organelle loses some of its genetic independence.

Many of the proteins made from these transferred genes are still used by the organelle. The genes are changed so they can work with nuclear transcription and translation machinery. They gain a promoter and a terminator, and a targeting sequence is added so the protein is sent back to the correct organelle. This lets the nucleus control protein supply and helps regulate organelle biogenesis, or organelle formation and development.

Some transferred genes also take on new roles. In Arabidopsis thaliana, almost 50% of plastid-derived genes carry out non-plastid functions. Scientists think gene transfer may protect vulnerable DNA from damage caused by free radicals made in mitochondria and chloroplasts. Another idea is that moving genes into the nucleus helps them avoid harmful mutation buildup described by Muller's ratchet, since nuclear genes can recombine during sexual reproduction.

How Organelles Pass Genes to the Nucleus

replication in prokaryotes

Prokaryotic DNA Replication Steps

Prokaryotic DNA replication copies the chromosome before a cell divides. In bacteria, this process starts at a specific DNA site called the origin of replication. From there, the cell builds new DNA strands using enzymes that match each base with its partner. The DNA double helix first opens so replication can begin. As the strands separate, each original strand serves as a template. New nucleotides are added in a set order, which helps the cell make an accurate copy of its genetic material. <br...

replication in eukaryotes

Eukaryotic DNA Replication Steps

Eukaryotic DNA replication copies the genome before a cell divides. In eukaryotes, this process happens in a controlled sequence so each new cell can receive genetic information. Replication begins at specific sites on the DNA called origins. The DNA is opened, and each original strand serves as a template, meaning it guides the building of a new complementary strand. This sets up two DNA molecules from one original molecule. The process then continues as enzymes add matching nucleotides to...

dna base pairing

DNA Base Pair Rules

DNA base pairing follows specific rules that help DNA carry genetic information. In DNA, the bases pair in a regular way, with adenine matching thymine and cytosine matching guanine. These pairs fit together because of their chemical shapes and bonding patterns. These base pairs also support accurate DNA replication. When DNA copies itself, each strand can guide the formation of a new partner strand using the pairing rules. Understanding base pairing helps explain how DNA stays organized and...

the dna replication fork

DNA Replication Stress and Fork Stalling

DNA replication stress and fork stalling can disrupt the copying of a cell’s genome. The replication fork is the Y-shaped site where DNA strands separate and new complementary strands are made at the same time. This linked unwinding and synthesis helps cells duplicate DNA efficiently and with fewer errors. In organisms with small circular DNA, such as E. coli, replication often begins at a single origin. That setup creates two replication forks that move in opposite directions from the opening...

proofreading

Proofreading in DNA Replication

Proofreading in DNA replication helps keep DNA copying accurate. It is a built-in check that reduces mistakes as new DNA is made. DNA polymerase is the enzyme that carries out this checking during replication. When it adds a wrong nucleotide, it can remove the error and replace it with the correct one. This correction step helps maintain the correct DNA sequence. Proofreading is an important part of DNA replication because it supports genetic accuracy. By catching mistakes as they happen, the...

dna helicases

Helicase Motor Proteins in DNA Unwinding

Helicase motor proteins help unwind DNA during key cell processes. They use energy from ATP hydrolysis, which is the breakdown of ATP to release usable energy. This movement lets helicases separate DNA strands when the cell needs access to the genetic code. These enzymes are involved in DNA replication, repair, recombination, and transcription. Helicases are found in all living organisms, but their structure, function, and movement can differ. In prokaryotes, the DnaB helicase moves along the...

lagging strand synthesis

Okazaki Fragments in DNA Replication

DNA replication uses a leading strand and a lagging strand, and they are made at different rates. The leading strand starts first. The lagging strand starts later, moves more slowly, and is made in pieces. The two strands also differ in direction and starting steps. Leading strand synthesis follows the opening replication fork. Lagging strand synthesis goes in the opposite direction. The leading strand needs one RNA primer, while the lagging strand needs multiple RNA primers. After priming,...

the replisome

DNA Replication Teams in the Replisome

DNA replication depends on the replisome, a coordinated complex of proteins that copies DNA with high accuracy. This DNA replication machinery brings many parts together so the leading and lagging strands can be made at the same time. A key idea in this process is the trombone model, proposed by Bruce Alberts in 1980. In this model, a loop forms on the lagging strand when a primer is made. The loop grows as an Okazaki fragment is synthesized, then falls apart when that fragment is finished.

mismatch repair

Fixing DNA Copying Errors

Mismatch repair helps fix DNA copying errors after replication. When DNA is copied, small mistakes can slip through. This repair system finds those mismatches and corrects them before they become permanent changes. The process depends on a group of mutator proteins. These proteins work together to detect the error, remove the incorrect section, and help replace it with the proper DNA sequence. In this way, mismatch repair supports the accuracy of genetic information. If the repair system does...

dna topoisomerases

How Topoisomerases Untwist DNA

DNA topoisomerases are enzymes that untwist DNA when it becomes overcoiled during cell processes such as DNA replication and transcription. They help regulate positive and negative DNA supercoiling without changing the nucleotide sequence. Clockwise overwinding creates positively supercoiled DNA, while counterclockwise underwinding produces negatively supercoiled DNA. Topoisomerases are grouped into two main types. Type I topoisomerases act on one strand of double-stranded DNA, and they...

telomeres and telomerase

How Telomerase Protects Chromosome Ends

Telomerase helps protect chromosome ends during eukaryotic DNA replication. When the final primer is removed, a single-stranded DNA fragment can remain at the end of the chromosome. That piece cannot be copied in the same way as the rest of the strand because there is no 3' end for new DNA to attach to. If this end piece is not replaced, chromosomal DNA is gradually lost each time a cell divides. The exposed single-stranded DNA can also trigger a DNA damage response from enzymes that detect it.

non-nuclear inheritance

Cytoplasmic Inheritance in Cells

Cytoplasmic inheritance explains how some traits are passed through cell structures outside the nucleus. These traits come from organelles such as mitochondria and chloroplasts. They do not follow the usual nuclear pattern of inheritance. Mitochondria and chloroplasts each contain their own DNA. Because of this, they can carry genetic information that affects cell function and certain visible traits. In high school biology, this topic helps explain why not all inherited traits come from...

animal mitochondrial genetics

Animal Mitochondrial DNA and Inheritance

Animal mitochondrial DNA has its own genome, separate from nuclear DNA. It is a double-stranded, closed circular molecule with about 20,000 base pairs. One strand is the heavy, or H-strand, because it is rich in guanine. The other is the light, or L-strand, because it is rich in cytosine. Compared with nuclear DNA, mitochondrial DNA has very little non-coding sequence. It also has no introns. Its genes are packed closely together, and some even overlap. The D-loop is the main non-coding...

comparing mitochondrial, chloroplast, and prokaryotic genomes

comparing-mitochondrial-chloroplast-and-prokaryotic-genomes

How Organelle Genomes Resemble Bacteria

Mitochondrial and chloroplast genomes still keep several traits from their bacterial ancestors. They also show changes that happened after these organelles evolved inside eukaryotic cells. Eukaryotic cells are cells with a nucleus, while prokaryotes are cells without one. Like prokaryotic genomes, mitochondrial and chloroplast genomes do not bind with histone-like proteins. They also do not show the complex chromosome-like packaging seen in eukaryotes. Mitochondria and chloroplasts divide by...

Eukaryotic cells can carry three genetic systems: nuclear, mitochondrial, and chloroplast. Over time, many genes from mitochondria and chloroplasts have moved into the nucleus. In some plant species, this gene transfer is still happening. This movement has affected large parts of the genome in some organisms. About 18% of the Arabidopsis thaliana nuclear genome is thought to come from the chloroplast's cyanobacterial ancestor. In yeast, around 75% of the genome is derived from the...

single-strand dna binding proteins

How SSB Proteins Protect DNA Strands

Single-strand DNA-binding proteins, or SSB proteins, help protect DNA during replication. When double-stranded DNA is unwound, the two strands separate. SSB proteins bind to the exposed single strands and keep them stable. These proteins bind in a sequence-independent manner. That means the order of the nitrogenous bases does not have to follow a specific pattern for SSB proteins to attach. Their binding straightens single-stranded DNA and makes it more rigid. This support may help DNA...

key terms and concepts

Learn how mitochondrial and chloroplast genes move into the nucleus, how proteins are sent back to organelles, and why this transfer may protect DNA.

How Organelles Pass Genes to the Nucleus | JoVE K12

molecular biology

JoVE Core Molecular Biology showcases the molecular processes of the cell through clear and concise animated video lessons. Additionally, in scientist-in-action videos, these concepts' application is demonstrated through real experiments in modern laboratories.

How Organelles Pass Genes to the Nucleus

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