3.4.3 Meiosis and Diversity (Unit 4 DNA and Relationships)

Question 1

Give 2 possible causes of variation that result from meiosis during gamete formation

Answer 1

• Random segregation of homologous chromosomes
• Crossing over and recombination of chromosomes
• (Mutation less common)

Question 2

Explain what happens to chromosomes in meiosis

Answer 2

• Chromosomes condense
• Chromosomes undergo independent segregation and arrange themselves in homologous pairs
• Crossing over and recombination of sections of chromosomes (this is rare)
• Chromosomes join to spindles at the middle of the cell joined by the centromere
• Homologous chromosomes are pulled apart to opposite poles (meiosis 1)
• The pairs or chromosomes are then separated at the centromere in meiosis 2 producing 4 genetically unique gametes.

Question 3

Describe how meiosis causes variation and explain the advantage of variation to the species

Answer 3

• Crossing over
• Independent assortment/segregation (Random assortment)

This generates new combinations of alleles

• Some members of the species will be better adapted so may survive if the environment changes
• So will be able to reproduce and pass on the advantageous alleles

Question 4

What is the biological importance of reducing the chromosome number when the cell divides by meiosis?

Answer 4

As the diploid number is restored during fertilisation

Question 5

State ways in which meiosis and mitosis are different

Answer 5

• Meiosis reduces the chromosome number where as mitosis maintains the chromosome number
• In meiosis chromosomes associate in pairs where as in mitosis chromosomes do not pair
• In meiosis there are 2 nuclear divisions that produce 4 gametes where as in mitosis there is 1 nuclear division producing 2 body cells
• Meiosis results in genetically unique daughter cells where as in mitosis the daughter cells are all genetically identical

Question 6

Briefly go through the stages of meiosis 1

Answer 6

1. Homologous chromosomes pair up randomly (independent segregation/assortment)
2. Chromatids from each pair wrap around each other then crossing over and recombination of portions of chromatids occurs.
3. One of each pair of chromosomes is randomly separated into 2 daughter cells.

Question 7

Briefly go through the stages of meiosis 2

Answer 7

1. Sister chromatids are pulled apart at the centromere by spindle fibres and separate into 4 haploid daughter cells
2. Resulting gametes have varying combinations of alleles

Question 8

Describe the role of the centromere in mitosis

Answer 8

It holds the chromatids together
It attaches the chromatids to the spindle and then splits the chromatids apart pulling them to separate poles of the cell

Question 9

Homologous chromosomes carry the same genes but are not genetically identical. Why?

Answer 9

Homologous chromosomes have different alleles

Question 10

Meiosis results in cells that have the haploid number of chromosomes and show genetic variation. How?

Answer 10

• Homologous chromosomes pair up
• Crossing over produces new combinations of alleles
• Chromosomes separate at random
• This produces varying combinations of chromosomes
• Chromatids then separate at meiosis 2

Question 11

Explain the role of independent segregation in meiosis

Answer 11

• To provide genetic variation.
• Allows different combinations of maternal and paternal chromosomes.
• Produces haploid cells.

Question 12

Identify the 2 ways that meiosis produces variation

Answer 12

crossing over

independent segregation

Question 13

Name an event that occurs during division 1 of meiosis that does not occur in division 2

Answer 13

pairing up of homologous pairs, crossing over followed by their separation to either pole

Question 14

How is mitosis similar to the second division of meiosis?

Answer 14

separation of sister chromosomes

Question 15

Define mutation.

Answer 15

Any change to the quantity of bases or the base sequence of DNA in an organism.

Question 16

Identify the 3 types of mutation that can occur.

Answer 16

1. Base substitutions
2. Base deletions
3. Chromosome mutations

Question 17

Describe how DNA nucleotide substitutions can lead to a non-functional protein.

Answer 17

1. A DNA nucleotide of a gene is replaced by a different nucleotide with a different base.
2. This change alters the DNA triplet and its resulting mRNA codon.
3. The altered codon translates into a different amino acid.
4. This results in a different primary structure.
5. Different hydrogen, ionic, disulphide bonds will form a new tertiary structure.
6. Protein with different shape results with different function/non-functional.

Question 18

Describe how DNA nucleotide deletions can lead to a non-functional protein.

Answer 18

1. A DNA nucleotide is lost from a normal DNA sequence.
2. This causes a ‘frame shift’ - each triplet after the deletion is shifted to the left by one base.
3. This alters the resulting mRNA codons.
4. The altered codons translate into different amino acids.
5. This results in a different primary structure.
6. Different hydrogen, ionic, disulphide bonds will form a new tertiary structure.
7. Protein with different shape results with different function/non-functional.

Question 19

Describe how DNA nucleotide substitution can lead to production of a functional protein.

Answer 19

1. A DNA nucleotide is swapped from a normal DNA sequence.
2. This alters the resulting mRNA codons.
3. But they still code for the same amino acid.
4. Primary structure is not altered, and the tertiary structure remains the same

or

1. A DNA nucleotide is swapped from a normal DNA sequence.
2. This alters the resulting mRNA codons.
3. But they are in an intron
4. So are removed by splicing
5. Correct protein is still formed

Question 20

What is a chromosome mutation?

Answer 20

Changes in the structure or number of whole chromosomes.

Question 21

What is ‘non-disjunction’?

Answer 21

1. Where homologous chromosomes fail to separate during meiosis 1.
2. This results in a gamete having too many or too few chromosomes e.g. Down’s Syndrome sufferers have an extra chromosome.

Question 22

What is the equation for calculating the possible chromosome combinations as a result of independent segregation during meiosis.

Answer 22

No. of chromosome combinations = 2^n

where n = the number of PAIRS of chromosomes

Question 23

What is the equation for calculating the possible chromosome combinations as a result of independent segregation during meiosis AND random fertilisation of gametes.

Answer 23

No. of chromosome combinations = (2^n)^2

where n = the number of PAIRS of chromosomes

Question 24

explain the difference in a cell that has under gone meiosis 1 and meiosis 2

Answer 24

meiosis 1 - only 2 nuclei/cells - only homologous chromosomes separated
meiosis 2 - 4 nuclei/cells - separation of sister chromosomes (haploid cells)

Question 25

How is chromosome number halved in meiosis

Answer 25

separation of homologous pairs

one of each pair ends up in the daughter cells

Question 26

In down syndrome, people have an extra chromosome (trisomy) - how does this happen?

Answer 26

In meiosis

homologous chromosomes/sister chormatids fail to separate

Question 27

define proteome

Answer 27

full range of proteins that a cell can produce/that the genome can code for

Question 28

Describe a brief overview of the events in meiosis

Answer 28

DNA is replicated in interphase before meiosis begins

In the first division there is a separation of homologous chromosomes, halving the chromosome number

In the second division there is separation of the sister chromatids

This produced four genetically different daughter cells

Question 29

How does meiosis lead to variation?

Answer 29

Mutations can occur changing the base sequence leading to the formation of new alleles

In the first division of meiosis there is crossing over where homologous chromosomes swap DNA producing new combinations of alleles

In meiosis homologous chromosomes may be independently segregated, separating into different daughter cells producing new combinations of alleles

During fertilisation there is random fusion of gametes this produces new combinations of alleles

Question 30

Define Genome

Answer 30

All the DNA in a cell

Question 31

Describe how a DNA mutation may lead to a shorter (truncated) protein being produced

Answer 31

1.Mutation changes the base sequence
2. One triplet is changed to a STOP CODON
3. Stop codon stops translation and the protein isn’t complete