💜 3.4.3 genetic diversity can arise as a result of mutation or during meiosis

Question 1

What is a mutation?

Answer 1

The spontaneous change in the DNA base sequences of chromosomes

Question 2

Which processes are known for mutations to be more common in and why?

Answer 2

Mutations occur spontaneously – especially during transcription and DNA replication. This is because the DNA base sequences are exposed.

Question 3

When does the rate of mutations increase?

Answer 3

On exposure to mutagenic agents. These include: ionising radiation from x-rays and from viruses.

Question 4

What are the three types of mutations?

Answer 4

1) substitution mutation
2) addition mutation
3) deletion mutation

Question 5

What is substitution mutation?

Answer 5

This is where one base in a nucleotide is replaced with another one. This might change the amino acid the triplet codes for, which would change the primary structure and therefore the shape of the protein, but it might not due to the degenerate nature of the code.

Question 6

What is addition mutation?

Answer 6

This is where an extra base is added to the DNA base sequence. This causes a framework shift downstream which means that all of the triplets after the mutation will change. Meaning that the primary structure will change. This will change the shape of the protein formed.

Question 7

What is deletion mutation?

Answer 7

This is where one of the bases is removed from the DNA base sequence. It causes a framework shift downstream similarly to an addition mutation.

Question 8

What is meiosis?

Answer 8

There is a type of cell division

Question 9

What does meiosis result in?

Answer 9

The production of non-identical cells – gametes

Question 10

What does meiosis produce in the offspring?

Answer 10

Variation

Question 11

What does meiosis do to the number of chromosomes in a cell?

Answer 11

It halves the number of chromosomes which means that the original chromosome number can be restored at fertilisation.

Question 12

What is a gamete?

Answer 12

Sex cell found in mammals in a sperm or egg. They fuse to become a Zygote.

Question 13

What is a diploid cell?

Answer 13

A cell which has two copies of each chromosome (2n)

Question 14

What is a haploid cell?

Answer 14

A cell which has one copy of each chromosome (n)

Question 15

What is a chromosome?

Answer 15

DNA wrapped with histone protein (only in eukaryotes)

Question 16

What is a chromatid?

Answer 16

Two of these are often held together by a centromere prior to cell division and they are copied during DNA replication to form chromosomes.

Question 17

What is a homologous chromosome?

Answer 17

Chromosomes containing the same genes for different alleles. One of each homologous pair comes from the paternal side and the other from the maternal side.

Question 18

What is an allele?

Answer 18

An allele is a different variation of the same gene

Question 19

How many stages are there in meiosis?

Answer 19

There are two stages

Question 20

What happens prior to meiosis and what does this result in?

Answer 20

DNA replication takes place and results in two initially identical sister chromatids joined together by a centromere

Question 21

Which stage of meiosis is largely ignored by Mark’s schemes and why?

Answer 21

Prophase as it is the same in both stages of meiosis and mitosis

Question 22

What are four sources of variation ?

Answer 22

1) Independence segregation, which is the order in which the homologous chromosomes line up on the equator
2) crossing over which results in different allelic combinations as the arms of the chromosomes wrap around each other and genetic information is exchanged
3) mutations which are spontaneous and can happen at any time
4) the random fusion of gamete’s at fertilisation creates genetic variation between zygotes as it will have a unique combination of alleles

Question 23

Describe what happens during metaphase one?

Answer 23

The chromosomes are all line up on the equator in double file. These homologous chromosomes are aligned via spindle fibres between the centromeres and centrioles.

Question 24

What happens during anaphase one?

Answer 24

The spindle fibres contract and each one of the homologous chromosomes is drawn to the opposite poles. They can be seen to be double structures and form a ‘V’ shape and then move upwards towards the poles.

Question 25

What happens during meiosis one - telophase (including cytokinesis)?

Answer 25

The chromosomes have reached the poles and the cytoplasm divides. At the end of telophase , two cells form - each with half the number of chromosomes as the original cell. The cells are now haploid.

Question 26

What happens during metaphase two?

Answer 26

The chromosomes now line upon the equator in single file. The spindle fibres (between the centrioles and the centromeres) align them on the equator.

Question 27

What happens during anaphase two?

Answer 27

As the spindle fibres contract, the sister chromatids are drawn to either pole and the centromeres divide

Question 28

What happens during meiosis two - telophase (including cytokinesis)?

Answer 28

The chromatids are now referred to as chromosomes and have reached the poles. The cytoplasm divides and four non-identical cells are formed. Although the mass of DNA halves in meiosis two, the number of chromosomes remains the same. These haploid cells are gametes.

Question 29

What is the point of meiosis?

Answer 29

- to make gametes which halves the number of chromosomes and therefore, after fertilisation restores the original diploid number - also, to increase variations by independent segregation and crossing over

Question 30

What are four differences between mitosis and meiosis?

Answer 30

1) there are two stages in meiosis whereas there is one stage in mitosis 2) mitosis produces genetically identical cells whereas meiosis produces genetically different cells 3) mitosis produces two cells whereas meiosis produces four cells 4) mitosis produces diploid cells whereas meiosis produces haploid gametes 5) there is crossing over and independence segregation in meiosis but there isn’t in mitosis

Question 31

Name one type of mutation, which is a chromosome mutation?

Answer 31

Non-disjunction mutation

Question 32

Where does non disjunction mutation occur and what is it?

Answer 32

It occurs in meiosis. It is either the non-separation of homologous chromosomes or sister chromatids and it results in a trisomy (three chromosomes in a cell rather than two)

Question 33

What are four types of variation?

Answer 33

1) mutations 2) random fusion of gametes 3) crossing over 4) independent segregation