🩶 3.4 Topic 4 - 3.4.3 Genetic diversity can arise as a result of mutation or during meiosis

Question 1

Durning fertilisation the sperm cell and the egg cell join together to form a _ _ _ _ _ _, which divides and develops into a new organism.

Answer 1

Zygote.

Question 2

Normal body cells have the diploid number (2n) of chromosomes, what does this mean?

Answer 2

Each cell contains two of each chromosome, one from the mum and one from the dad.

Question 3

Gametes (sperm and egg cells) have a haploid (n) number of chromosomes, what does this mean?

Answer 3

There’s only one copy of each chromosome.

Question 4

During fertilisation a diploid number (2n) of chromosomes is formed? Why?

Answer 4

A haploid sperm fuses with a haploid egg.

Question 5

Random fertilisation is a term used to describe fertilisation. Why?

Answer 5

Because fertilisation is random.

Question 6

What is random fertilisation?

Answer 6

It is where zygotes are produced with different combinations of chromosomes to both parents. The mix of the genetic material in sexual reproduction increases genetic diversity within a species.

Question 7

Are gametes formed by meiosis?

Answer 7

Yes.

Question 8

Cells that divide by meiosis are _ _ _ _ _ _ _ to start with, but the cells that are formed from meiosis are _ _ _ _ _ _ _. Why?

Answer 8

Diploid.
Haploid.
This is because the chromosome number is halved so when the gametes fuse you don’t get double the number of chromosomes as this is not good.

Question 9

Describe the process of meiosis.

Answer 9

1) DNA unravels and replicates so there is two copies of each chromosome, called chromatids.
2) DNA condenses to form double-armed chromosomes, each made from two sister chromatids, joined in the middle by a centromere.
3) Meiosis I - chromosomes arrange into homologous pairs and separate, halving the chromosome number.
4) Meiosis II - The pair of sister chromatids that make up each chromosome are separated as the centromere is divided.
5) Four haploid cells (gametes) which are genetically different from each other are produced.

Question 10

Humans have _ _ chromosomes and in total _ _ pairs.

Answer 10

46.
23.

Question 11

What are homologous pairs of chromosomes?

Answer 11

They are two chromosomes, 1 from mum and 1 from dad, which are the same size, have the same genes, although they could have different versions of those genes (alleles).

Question 12

Do chromatids cross over in meiosis I or II?

Answer 12

I.

Question 13

How does crossing over occur?

Answer 13

Homologous chromosomes come together and pair up. The chromatids twist around each other and bolts of chromatids swap over. The chromatids still contain the same genes but now have a different combination of alleles.

Question 14

What does crossing over produce?

Answer 14

A different combination of alleles. It means that each of the four daughter cells formed from meiosis contains chromatids with different alleles.

Question 15

What are the two main events in meiosis which lead to genetic variation?

Answer 15

Crossing over of chromatids.
Independent segregation of chromosomes.

Question 16

Does independent segregation happen in meiosis I or II?

Answer 16

I.

Question 17

What is independent segregation?

Answer 17

It is where the homologous pairs are separated and this is completely random of which chromosome from each pair ends up in which daughter cell. The four daughter cells produced by meiosis have completely different combinations of the maternal and paternal chromosomes.

Question 18

What does meiosis produce?

Answer 18

Cells with HALF the number of chromosomes as the parent cell. The daughter cells are genetically DIFFERENT from one another and the parent cell. 4 daughter cells are produced.

Question 19

What does mitosis?

Answer 19

Cells with the SAME number of chromosomes as the parent cell. The daughter cells are genetically IDENTICAL to each other and to the parent cell. 2 daughter cells are produced.

Question 20

How many divisions are in meiosis?

Answer 20

2 - which separate the homologous pairs and then the sister chromatids.

Question 21

How many divisions are in mitosis?

Answer 21

1 - which separates the the sister chromatids.

Question 22

Why are the two daughter cells produced in mitosis, genetically identical?

Answer 22

Because there is no pairing or separating of homologous chromosomes and therefore no crossing over or independent segregation of chromosomes.

Question 23

When meiosis works, all 4 daughter cells will end up with _ _ whole chromosomes - one from each homologous pair.

Answer 23

23.

Question 24

What is chromosome mutation and what is it caused by?

Answer 24

The cells produced contain variations in the numbers of whole chromosomes or parts of chromosomes.
It is caused by errors during meiosis.

Question 25

What can chromosome mutations lead to?

Answer 25

Inherited conditions because the errors are present in the gametes (hereditary cells).

Question 26

Non-disjunction is a type of chromosome mutation.
What does non-disjunction mean?

Answer 26

It’s a failure of the chromosomes to separate properly. In humans, non-disjunction of chromosome 21 during meiosis can lead to down’s syndrome.

Question 27

What is down’s syndrome caused by?

Answer 27

By a person having an extra copy of chromosome 21 (or sometimes an extra copy of part of chromosome 21)

Question 28

In down’s syndrome, what does non-disjunction mean? and what does it mean when fertilisation happens?

Answer 28

It means that chromosome 21 fails to separate properly during meiosis, so one cell gets an extra copy of 21 and another gets non.
When the gamete with an extra copy fuses to another gamete at fertilisation, the resulting zygote will have three copies of chromosome 21.

Question 29

When can non-disjunction also occur?

Answer 29

In meiosis II.

Question 30

What do gene mutations involve?

Answer 30

A change in the DNA base sequence of chromosomes.

Question 31

What are the two types of gene mutations that can occur?

Answer 31

Substitution and deletion.

Question 32

What happens during the substitution gene mutation?

Answer 32

One base is substituted with another, e.g. ATGCCT becomes ATTCCT (G swapped for T).

Question 33

What happens during the deletion gene mutation?

Answer 33

One base is deleted, e.g. ATGCCT becomes ATCCT (G is deleted).

Question 34

What does the order of DNA bases in a gene determine?
and what happens if a mutation occurs in a gene?

Answer 34

The order of amino acids in a particular protein.
The sequence of amino acids it codes for (and the protein formed) could be altered.

Question 35

How do not all mutations affect the order of amino acids?

Answer 35

Because the genetic code is degenerate. This means that some amino acids are coded for by more than one DNA triplet. Therefore, not all substitution mutations will result in change to the amino acid sequence of the protein - some substitutions will still code for the same amino acid.

Question 36

Substitution mutations will not always lead to changes in the amino acid sequence, however deletions will. Why do deletion mutations lead to changes in the amino acid sequence?

Answer 36

The deletion of bases will change the number of bases present, which will cause a shift in all the base triplets after it.

Question 37

What do mutagenic agents do?

Answer 37

They increase the rate of mutation.

Question 38

Mutations occur S _ _ N _ _ _ _ O _ _ L _.

Answer 38

Spontaneously.
E.g. when DNA is misread during replication.

Question 39

What are some examples of mutagenic agents?

Answer 39

Ultraviolet radiation, ionising radiation, some chemical and some viruses.