Mutations,meiosis and genetic variation

Question 1

What is a gene mutation

Answer 1

Any change to one or more nucleotide bases, or a change in the sequence of the bases in DNA

Question 2

What are the three key types of gene mutation

Answer 2

Insertion
Substitution
Deletion

Question 3

Describe what happens when a gene mutation by substitution occurs

Answer 3

Substitution refers to the type of gene mutation in which a nucleotide in a DNA molecule is replaced by another nucleotide that has a different organic base.

Question 4

Explain why substitution (type of gene mutation) can have detrimental affects

Answer 4

• The base sequence of amino acids codes for a sequence of amino acids that then make upon a protein.
• If one organic base in the sequence is substituted for another, this could code for a different amino acid.
• The polypeptide produced will differ in a single amino acid.
• The significance of this difference will depend upon the precise role of the original amino acid.
• If it is important in forming bonds that determine the tertiary structure of the final protein, then the replacement amino acid may not form the same bonds.
• The protein may then be a different shape and so may not function properly (eg. If the protein was an enzyme)

Question 5

Explain why some substitution mutations can have no effect

Answer 5

• The DNA code is degenerate.
• Most amino acids have more than one codon and as a result of this, a base can be substituted for a different one but the triplet can still code for the same amino acid.
• If the same amino acid is coded for, the polypeptide will be unchanged and so the mutation will have no effect.

Question 6

Describe what happens when a gene mutation by deletion occurs

Answer 6

A nucleotide is lost from the DNA sequence.

Question 7

Explain why a gene mutation by deletion has detrimental affects

Answer 7

• the loss of a single nucleotide in a DNA sequence causes the amino acid sequence of the polypeptide to be completely different and so it is unlikely to function correctly.
• This is because the sequence of bases in DNA is read in units of three bases (triplet).
• One deleted nucleotide causes all triplets in a sequence to be read differently because each has been shifted to the left by one base.

Question 8

What are chromosome mutations

Answer 8

Changes in the structure or number of whole chromosomes

Question 9

What two forms can chromosome mutations take

Answer 9

• Changes in whole sets of chromosomes.
• Changes in the number of individual chromosomes.

Question 10

Describe the type of chromosome mutation where there are changes to the number of whole sets of chromosomes

Answer 10

Changes in whole sets of chromosomes occur when organisms have three or more sets of chromosomes rather than the usual two. This condition is called polyploidy and occurs mostly in plants.

Question 11

Describe the type of chromosome mutation where there are changes in the individual number of chromosomes

Answer 11

• Sometimes individual homologous pairs of chromosomes fail to separate during meiosis.
• This is known as non-dysjunction and usually results in a gamete having either one more or one fewer chromosome.
• on fertilisation with a gamete that has the normal complement of chromosomes, the resultant offspring have more or fewer chromosomes than normal in all their body cells.
• An example of non-dysjunction in humans is Down’s syndrome, where individuals have an extra chromosome 21

Question 12

What are the two ways in which cell division occurs

Answer 12

Mitosis and meiosis

Question 13

Describe the key difference in chromosome number between mitosis and meiosis

Answer 13

• Mitosis produces two daughter cells with the same number of chromosomes as the parent cell and as each other.
• Meiosis usually produces four daughter cells, each with half the number of chromosomes as the parent cell.

Question 14

What happens during sexual reproduction

Answer 14

Two gametes fuse to give rise to new offspring that are genetically different from the parent and each other.

Question 15

What are gametes

Answer 15

Sex cells (sperm and egg in humans) that contain half the number of chromosomes that a usual cell does. This is the haploid number.

Question 16

What is the haploid number

Answer 16

The number of chromosomes found in gametes which is half the amount found in a normal body cell

Question 17

What is the diploid number

Answer 17

The number of chromosomes found in a normal cell- so diploid cells have a full set of chromosomes.

Question 18

Why do gametes only contain half the number of chromosomes that a normal cell does

Answer 18

• If each gamete had a full set of chromosomes, then the cell that they produce would have double this number.
• This doubling of the number of chromosomes would continue at each generation.
• In order to maintain a constant number of chromosomes in adults of a species, they must be halved at some point in the life cycle.
• So when two haploid gametes fuse during fertilisation, the diploid number of chromosomes is restored.

Question 19

What process halves the number of chromosomes at some point in the life cycle

Answer 19

Meiosis

Question 20

When does meiosis occur in most animals

Answer 20

In the formation of gametes

Question 21

What is a gene

Answer 21

A length of DNA that codes for a polypeptide

Question 22

What is a locus (genetics)

Answer 22

The position of a gene on a chromosome or DNA molecule.

Question 23

What is an allele

Answer 23

One of the different forms of a particular gene.

Question 24

What are homologous chromosomes

Answer 24

A pair of chromosomes, one maternal and one paternal, that have the same gene loci.

Question 25

What two key things does meiosis achieve

Answer 25

Halves the number of chromosomes and produces genetic variation amongst the offspring.

Question 26

How/in what ways does meiosis bring about genetic variation

Answer 26

• independent segregation of homologous chromosomes.
• new combinations of maternal and paternal alleles by crossing over.

Question 27

How many divisions are involved in meiosis

Answer 27

Meiosis involves two nuclear divisions that usually occur one after another.

Question 28

Summarise what happens during each of the two nuclear divisions in meiosis

Answer 28

1) In the first division (meiosis 1) homologous chromosomes pair up and their chromatids wrap around each other. Equivalent portions of these chromatids may be exchanged in a process called crossing over. By the end of this division the homologous pairs have separated, with one chromosome from each pair going into one of the two daughter cells.
2) In the second meiotic division (meiosis 2) the chromatids move apart. At the end of meiosis 2, four cells have usually been formed. In humans each of these cells contains 23 chromosomes (gametes)

Question 29

Describe what independent segregation is (of homologous chromosomes)

Answer 29

• During meiosis 1, each chromosome lines up alongside its homologous partner.
• When these homologous pairs arrange themselves in this line, they do so at random.
• One of each pair will pass to each daughter cell. Which one goes to each daughter cell depends on how the pairs are lined up.
• Since the pairs are lined up at random, the combination of chromosomes of maternal and paternal origin that go into the daughter cell at meiosis 1 is a matter of chance- this is independent segregation.

Question 30

Explain how genetic recombination by crossing over increases genetic variety

Answer 30

• If there is no genetic recombination by crossing over, only two different types of cell are produced.
• However, if there is genetic recombination, four different types of cell are produced.
• Crossing over therefore increases genetic variety.

Question 31

Describe the process of genetic recombination by crossing over

Answer 31

• During meiosis 1, each chromosome lines up alongside its homologous partner.
• Then the chromatids of each pair become twisted around each other.
• During this twisting process, tensions are created and portions of the chromatids break off.
• These broken portions might then rejoin with the chromatids of its homologous partner.
• Usually it is the equivalent portions of homologous chromosomes that are exchanged
• In this way new genetic combinations of maternal and paternal alleles are produced.
• The chromatids pass over each other many times- so the process is crossing over.
• The broken-off portions of chromatid recombine with another chromatid hence recombination.

Question 32

Describe how genetic variety is achieved via new genetic combinations

Answer 32

• Each member of a homologous pair of chromosomes has exactly the same genes and so determines the same characteristics.
• However, the alleles of these genes might differ.
• The independent assortment of these chromosomes therefore produces new genetic combinations.
• Where the cells produced in meiosis are gametes, they will be genetically different.
• Then, when these gametes fuse randomly at fertilisation, variety is produced in the offspring.

Question 33

What formula do we use to determine the number of possible combinations of chromosomes for each daughter cell (produced during meiosis) when the parent cell/gamete are from one/the same parent/

Answer 33

2^n where n=the number of pairs of homologous chromosomes.

Question 34

What formula do we use to calculate the number of possible chromosomes combinations in each daughter cell when the gametes came from two different parents.

Answer 34

(2^n)^2 Where n= the number of pairs of homologous chromosomes.

Question 35

What is the problem with the formulas we use for calculating the number of possible chromosome combinations during meiosis.

Answer 35

• They assume that the chromosomes stay intact throughout meiosis.
• However, crossing over occurs during meiosis 1 and this exchanges sections of chromosomes between homologous pairs in a process called recombination.
• As recombination occurs each time gametes are made, it will greatly increase the number of possible chromosomes combinations in the gametes.