Genetic Diversity: Mutations & Meiosis (AQA A Level Biology): Flashcards
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Define the term gene mutation
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Define the term gene mutation
A gene mutation is a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide.
When and how often do gene mutations arise?
Gene mutations arise spontaneously during DNA replication.
They occur continuously and at random.
Define a base substitution mutation
A base substitution is a mutation in which a base in the DNA sequence is randomly swapped for a different base.
Define a base deletion mutation
A base deletion is a mutation in which a nucleotide (and therefore its base) is randomly deleted from the DNA sequence.
Why do not all base substitutions change the amino acid sequence of a polypeptide?
The genetic code is degenerate.
This means more than one codon can code for the same amino acid, so a substitution may be silent and not alter the amino acid sequence.
State the three possible outcomes of a base substitution mutation
The mutation may be silent, not altering the amino acid sequence.
The mutation may change a single amino acid in the polypeptide (e.g. sickle cell anaemia).
The mutation may create a premature stop codon, producing an incomplete polypeptide.
Why does a base deletion often have a greater effect on a polypeptide than a base substitution?
A deletion causes a frameshift.
It has a knock-on effect, changing every triplet further on in the DNA sequence.
This may dramatically change the amino acid sequence and the polypeptide's ability to function.
Define a mutagenic agent
A mutagenic agent is an environmental factor that increases the mutation rate of cells.
Give three examples of mutagenic agents
High-energy radiation, such as UV light.
Ionising radiation, such as X-rays.
Toxic chemicals, such as peroxides.
Explain how a gene mutation can affect a protein's function
A small number of mutations code for a significantly altered polypeptide with a different shape.
If the shape of an enzyme's active site changes, the substrate may no longer be able to bind.
A structural protein (like collagen) may lose its strength if its shape changes.
Most gene mutations do not alter the polypeptide because the genetic code is , meaning a substitution can be described as when it does not change the amino acid sequence.
Most gene mutations do not alter the polypeptide because the genetic code is degenerate, meaning a substitution can be described as silent when it does not change the amino acid sequence.
Define non-disjunction
Non-disjunction occurs when chromosomes fail to separate during meiosis.
What is a chromosome mutation and how does it arise?
A chromosome mutation is a change in the number of chromosomes.
It arises spontaneously by non-disjunction during meiosis.
How does non-disjunction affect the chromosome number of gametes?
The gametes may end up with one extra copy of a particular chromosome or no copies of that chromosome.
These gametes have a different number of chromosomes compared to the normal haploid number.
How does an abnormal gamete lead to a chromosome mutation at fertilisation?
If the abnormal gamete takes part in fertilisation, the resulting diploid cell will have the incorrect number of chromosomes.
Using Down's syndrome as an example, describe a chromosome mutation
Individuals with Down's syndrome have a total of 47 chromosomes instead of 46.
This is because they have three copies of chromosome 21.
At which stage of the life cycle does the non-disjunction leading to a chromosome mutation occur?
During meiosis, when the gametes are being produced.
True or False?
Non-disjunction occurs spontaneously during meiosis.
True.
Non-disjunction occurs spontaneously when chromosomes fail to separate during meiosis.
True or False?
A person with Down's syndrome has two copies of chromosome 21.
False.
A person with Down's syndrome has three copies of chromosome 21, giving a total of 47 chromosomes.
Chromosome mutations involve , which occurs when chromosomes fail to separate during .
Chromosome mutations involve non-disjunction, which occurs when chromosomes fail to separate during meiosis.
Name the two processes during meiosis that produce genetically different daughter cells
Independent segregation of homologous chromosomes.
Crossing over between homologous chromosomes.
Define independent segregation of homologous chromosomes
Independent segregation is where each pair of homologous chromosomes lines up randomly during meiosis I, so the way one pair segregates does not affect how another pair segregates.
During which division of meiosis does independent segregation occur, and what happens?
It occurs during meiosis I.
Homologous chromosomes line up in the centre of the cell and are then separated and pulled into different cells.
Define crossing over
Crossing over is the process where homologous chromosomes exchange genetic material during meiosis I.
Describe the process of crossing over
Homologous chromosomes pair up and form bivalents.
The chromatids break and rejoin to the chromatid of the homologous chromosome, so that alleles are exchanged.
This produces new combinations of alleles, known as recombination.
Define recombination in the context of meiosis
Recombination is the production of new combinations of alleles on chromatids as a result of crossing over.
Why is independent segregation described as "independent"?
Because each pair of homologous chromosomes lines up randomly.
The way one pair segregates does not affect the way another pair segregates — each pair takes a different path by chance.
True or False?
Crossing over occurs during meiosis I.
True.
Homologous chromosomes pair up and exchange genetic material during meiosis I.
Independent segregation and both result in different combinations of in the daughter cells produced by meiosis.
Independent segregation and crossing over both result in different combinations of alleles in the daughter cells produced by meiosis.
Define meiosis
Meiosis is a form of nuclear division that produces haploid cells from diploid cells.
It produces the gametes used in sexual reproduction.
How many divisions does meiosis involve, and what are they called?
Meiosis involves two divisions: meiosis I and meiosis II.
What does meiosis I achieve?
Meiosis I separates the homologous chromosomes.
It halves the chromosome number, producing two haploid cells.
Describe the main events of prophase I
DNA condenses and becomes visible as chromosomes.
Chromosomes arrange side by side in homologous pairs.
Crossing over of non-sister chromatids may occur.
The nuclear envelope breaks down and the spindle forms.
What happens during metaphase I?
Homologous pairs of chromosomes line up randomly along the equator of the spindle.
Independent segregation occurs.
What happens to the chromosomes during anaphase I?
Whole chromosomes are pulled to opposite poles of the spindle as the homologous pairs are separated.
The centromeres do not divide.
How does anaphase II differ from anaphase I?
In anaphase II, the centromeres divide.
Individual chromatids are pulled to opposite poles, each now becoming an individual chromosome.
Is DNA replicated between meiosis I and meiosis II?
No.
There is no interphase between meiosis I and meiosis II, so the DNA is not replicated.
What is the final product of meiosis?
Four haploid cells.
Each has half the chromosome number of the original diploid parent cell.
True or False?
In prophase II, a spindle forms at a right angle to the old one.
True.
During prophase II the nuclear envelope breaks down, the chromosomes condense, and a spindle forms at a right angle to the old one.
Chromosomes are counted by their , so after DNA replication a human cell still has 46 chromosomes but a total of 92 .
Chromosomes are counted by their centromeres, so after DNA replication a human cell still has 46 chromosomes but a total of 92 chromatids.
State the three sources of genetic variation associated with sexual reproduction
Independent segregation of homologous chromosomes.
Crossing over between homologous chromosomes.
Random fertilisation of gametes.
Explain how random fertilisation creates genetic variation
During fertilisation, any male gamete can fuse with any female gamete.
This random fusion produces zygotes that each have a unique combination of alleles.
What formula gives the number of possible chromosome combinations following meiosis (without crossing over)?
2^n^
Where n is the number of homologous chromosome pairs (the haploid number).
How many chromosome combinations can result from meiosis in humans, where n = 23?
2^23^ = 8 388 608 possible chromosomal combinations.
What formula gives the number of chromosome combinations following the random fertilisation of two gametes?
(2^n^)^2^
Where n is the haploid number and the power of 2 represents the two gametes fusing.
A plant species has a diploid number of 16. Calculate the number of chromosome combinations that can result from meiosis (no crossing over).
Step 1: Calculate the haploid number: n = 16 ÷ 2 = 8
Step 2: Apply the formula 2n = 2^8^
Step 3: 28 = 256 different chromosomal combinations.
Why can siblings with the same parents be genetically distinct?
Because of variation introduced during meiosis (independent segregation and crossing over) and at random fertilisation.
There is almost zero chance that offspring from successive sexual reproduction are genetically identical.
True or False?
The random fusion of gametes at fertilisation increases genetic variation between zygotes.
True.
Any male gamete can fuse with any female gamete, so each zygote has a unique combination of alleles.
The number of possible chromosome combinations following meiosis is equal to , where n is the number of homologous chromosome .
The number of possible chromosome combinations following meiosis is equal to 2^n^, where n is the number of homologous chromosome pairs.
How can you identify that a cell is in meiosis I from a photomicrograph?
Homologous chromosomes pair up side by side in meiosis I only.
So if pairs of chromosomes are visible, meiosis I must be occurring.
How does the number of cells formed help distinguish meiosis I from meiosis II?
If two new cells are formed, it is meiosis I.
If four new cells are formed, it is meiosis II.
What might you observe about the chromosomes during meiosis II?
Single chromosomes (not homologous pairs) may be observed.
What would you observe in a cell at metaphase I?
Homologous pairs of chromosomes lined up side by side along the equator of the spindle.
What would you observe in a cell at metaphase II?
Single whole chromosomes lined up in a single file along the equator of the spindle.
What would you observe in a cell at anaphase I?
Whole chromosomes being pulled to opposite poles, with the centromeres intact.
What would you observe in a cell at telophase II?
Nuclei forming around four groups of condensed chromosomes.
True or False?
Homologous pairs of chromosomes are visible during meiosis II.
False.
Homologous chromosomes pair up in meiosis I only. During meiosis II, single chromosomes are observed.
If pairs of chromosomes are visible, the cell is in meiosis , whereas the formation of four new cells indicates meiosis .
If pairs of chromosomes are visible, the cell is in meiosis I, whereas the formation of four new cells indicates meiosis II.
How many daughter cells are produced by mitosis and by meiosis?
Mitosis produces two daughter cells.
Meiosis produces four daughter cells.
Compare the genetic makeup of the daughter cells produced by mitosis and meiosis
Mitosis produces cells that are genetically identical to each other and to the parent cell.
Meiosis produces cells that are genetically different from each other and from the parent cell.
Compare the ploidy of the daughter cells produced by mitosis and meiosis
Mitosis produces diploid cells (46 chromosomes).
Meiosis produces haploid gametes (23 chromosomes).
How many times does the cell divide in mitosis compared with meiosis?
In mitosis, the cell divides once.
In meiosis, the cell divides twice.
State the function of mitosis and of meiosis
Mitosis is used for growth and repair.
Meiosis is used for the production of gametes.
Where in the body do mitosis and meiosis occur?
Mitosis occurs all over the body.
Meiosis occurs in the sex organs.
Why is meiosis important for a species?
Meiosis produces genetic variation within a species.
This variation is the raw material for natural selection, helping the species adapt to a changing environment.
How can you recognise where meiosis occurs in an unfamiliar life cycle?
Meiosis always involves a reduction division.
When the ploidy of the cell is halved, meiosis has just occurred.
True or False?
Mitosis produces daughter cells that are genetically identical to the parent cell.
True.
Mitosis produces two daughter cells that are genetically identical to each other and to the parent cell.
In mitosis the cell divides once to produce two cells, whereas in meiosis the cell divides twice to produce four cells.
In mitosis the cell divides once to produce two diploid cells, whereas in meiosis the cell divides twice to produce four haploid cells.
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