Unit 4 - Meiosis + Mutation

Question 1

What is a chromosome?

Answer 1

An independent DNA molecule.

Question 2

What is a gene?

Answer 2

A base sequence of DNA that codes for a polypeptide or functional RNA.

Question 3

What is an allele?

Answer 3

Different forms of the same gene.

Question 4

What is a locus?

Answer 4

The fixed position of a gene on a particular chromosome.

Question 5

Definition of homologous chromosomes?

Answer 5

A pair of chromosomes which have the same genes at the same gene loci but may have different alleles.

Question 6

What is a haploid cell?

Answer 6

A cell containing one of each type of chromosome. (n)

Question 7

What is a diploid cell?

Answer 7

A cell containing 2 of each type of chromosome as pairs (2n)

Question 8

What is a gamete?

Answer 8

A sex cell (n) containing one of each type of chromosome, in humans, the egg and sperm.

Question 9

What is sexual reproduction?

Answer 9

When 2 gametes (n) join to make a zygote (the first cell (2n) of a new organism). The joining of the gametes is called fertilisation.

Question 10

What does meiosis result in?

Answer 10

4 haploid daughter cells that are genetically different from each other from a single diploid parent cell.

Question 11

What are the key points of meiosis?

Answer 11

-Homologous chromosomes pair, forming a bivalent, and line up along the equator of the cell, through independent segregation - different combinations of maternal and paternal chromosomes results.
-Crossing over occurs, resulting in different combinations of alleles.
-During the first division, homologous chromosomes separate and are pulled apart by their centromeres to opposite poles of the cell.
-During the 2nd division of meiosis, sister chromatids separate and are pulled apart by their centromeres to opposite poles of the cell.

Question 12

How does crossing over of homologous chromosomes occur?

Answer 12

The homologous chromosomes come together as a pair, forming a bivalent. Chromatids on different chromosomes, so non-sister chromatids, cross over. The point of cross over is called a chiasma. Equal lengths of non-sister chromatids are exchanged. The resulting chromatids contain the same genes but different combinations of alleles.

Question 13

How does independent segregation of chromosomes occur?

Answer 13

When homologous chromosomes line up in the equator they do so completely randomly. Once they separate during the first meiotic division, it is completely random which chromosome from each pair ends up in which daughter cell. So the 4 daughter cells produced by meiosis from the original cells have different combinations of those parental chromosomes (but still 1 of each type).

Question 14

How does random fertilisation of haploid gametes occur?

Answer 14

During fertilisation, any sperm can fertilise any egg. This results in the potential for many different combinations of pairs of chromosomes in the resulting zygote (but still 2 of each type of chromosome in each pair).

Question 15

Differences between meiosis and mitosis?

Answer 15

Meiosis: -Diploid parent cells produce haploid daughter cells.
-Homologous chromosomes associate in pairs on the equator before the 1st division.
-Crossing over of homologous chromosomes occurs.
-There is a total of 2divisions/4daughter cells produced.
Mitosis: -Diploid=Diploid Haploid=Haploid
-Homologous chromosomes do not pair together on the equator.
-No crossing over of chromosomes.
-1 division/2daughter cells produced
-Genetically identical cells produced.

Question 16

Why is meiosis important?

Answer 16

Halving the number of chromosomes in meiosis to give haploid cells is important, so that when fertilisation occurs, the diploid number of chromosomes is restored in the cell and the chromosome number is not doubled.

Question 17

Definition of a gene mutation?

Answer 17

A change in the DNA base sequence of chromosomes.

Question 18

What are mutagenic agents?

Answer 18

Something that can cause an increase in the rate of gene mutation.

Question 19

When do mutations occur?

Answer 19

They can arise spontaneously during DNA replication.

Question 20

What is a substitution mutation?

Answer 20

One base is swapped with another different base.

Question 21

What is the effect of a substitution mutation?

Answer 21

Sometimes none because the degenerate nature of the genetic code, some amino acids are coded for by more than one DNA triplet.

Question 22

What is a deletion mutation?

Answer 22

One base is removed from the sequence.

Question 23

What is the effect of a deletion mutation?

Answer 23

Always an effect - a deleted nucleotide results in an alteration of the base triplets from the mutation onwards. The reading frame has been shifted to the left by 1 letter and is known as a frame shift. All triplets from the point of mutation coding for the polypeptide could be altered, so all the amino acids from the point of mutation will hr altered.

Question 24

What happens when a mutation causes a triplet to code for a stop codon?

Answer 24

This will cause the growing polypeptide chain to terminate prematurely, and would probably not be able to perform its proper function (depending on how early the termination occurs).

Question 25

What happens if a mutation causes the DNA sequence to change?

Answer 25

-The sequence of bases in DJA and so the mRNA is altered
-The amino acid sequence coded for alters (primary structure)
-The hydrogen, disulphide and ionic bonds form in different places
-The protein folds into a different shape as the tertiary structure changes
-The protein is non-functional
-If it is an enzyme… different active site shape means that it is no longer complementary to the substrate - no enzyme substrate complexes can form.

Question 26

What happens if a mutation causes the DNA sequence to change?

Answer 26

-The sequence of bases in DJA and so the mRNA is altered
-The amino acid sequence coded for alters (primary structure)
-The hydrogen, disulphide and ionic bonds form in different places
-The protein folds into a different shape as the tertiary structure changes
-The protein is non-functional
-If it is an enzyme… different active site shape means that it is no longer complementary to the substrate - no enzyme substrate complexes can form.

Question 27

How many chromosomes are in a gamete cell?

Answer 27

23 chromosomes

Question 28

What is chromosome non-disjunction?

Answer 28

A type of chromosome mutation, when chromosomes fail to separate properly during meiosis. This can take place during the 1st division (homologous chromosomes fail to separate) or 2nd division (sister chromatids fail to separate).

Question 29

What do chromosome mutations lead to?

Answer 29

Inherited conditions because there are the wrong number of chromosomes in the resulting gametes, I.e. down-syndrome