Chapter 3- Meiosis

Question 1

Why is meiosis known as reduction division?

Answer 1

Because it involved halving the chromosome number

Question 2

What does the halving of chromosomes allow?

Answer 2

Allows a sexual life cycle with fusion of gametes

Question 3

What is fertilisation?

Answer 3

Is the Union of sex cells, or gametes, usually from different parents

Question 4

What happens just before meiosis?

Answer 4

DNA is replicated so that all chromosomes consist of two sister chromatids

Question 5

What do the early stages of meiosis involve?

Answer 5

Pairing of homologous chromosomes and crossing over followed by condensation

Question 6

First couple of stages of meiosis

Answer 6

1) homologous chromosomes pair up with each other
2) because DNA replication has already occurred, each chromosome consists of two chromatids and so there are four DNA molecules associated in each pair of homologous chromosomes
3) a pair of homologous chromosomes is bivalent and the pairing process is sometimes called synapsids

Question 7

What happens after synapsis?

Answer 7

Process called crossing over takes place - outcome is that a junction is created where one chromatid in each homologous breaks up and rejoins win the other chromatid

Question 8

What is crossing over?

Answer 8

Occurs random positions anywhere along the chromosomes

Question 9

The orientation of homologous chromosomes is what?

Answer 9

Prior to separation is random which means meiosis creates genetically diverse haploid cells

Question 10

What is disjunction?

Answer 10

Two chromosomes in each bivalent are held together by chiasmata but these slide to the end of the chromosomes and then the chromosomes can separate - one chromosome from each bivalent moves to one of the poles and the other chromosomes to the other pole

Question 11

What does disjunction do?

Answer 11

Half the chromosomes- which means the first division is the reduction devision

Question 12

What are two methods of obtaining cells for karyotype?

Answer 12

Chronic villus sampling and amniocentesis

Question 13

What is aminocentasis?

Answer 13

Involves the passing of a needle through the mothers abdomen- the needle is used to withdraw a sample of amniotic fluid containing feral cells from the amniotic sac

Question 14

What is chorionic villus?

Answer 14

A sampling tool that enters through the vagina is used to obtain cells from the chorion, one of the membranes from which the placenta developed

Question 15

What is the risk of miscarriage from the 2 ways of obtaining cells a?

Answer 15

Aminocentasis = 1%

Chronic villus = 2%

Question 16

Explain the first division of meiosis

Answer 16

Prophase I : 1) homologous chromosomes pair (snyapsis)
2) crossing over occurs

Metaphase I : 1) microtubules move homologous pairs to equator of cell
2) orientation of paternal and maternal is random

Anaphase I: homologous pairs are separated one chromosome of each pair moves to each pole

Telophase I: 1) chromosome uncoil

2) reduction of chromosome number from diploid to haploid completed
3) cytokinesis occurs

Question 17

Explain the second devision of meiosis

Answer 17

Prophase II: chromosome which still consists of two chromatids, condense and become visible

Metaphase II

Anaphase II: centromeres separate and chromatids are moved to opposite poles

Telophase II: 1) chromatids reach opposite poles

2) nuclear envelope forms
3) cytokinesis occurs

Question 18

What does crossing over and random orientation promote ?

Answer 18

Genetic variation

Question 19

How does random orientation of bivalent sallow genetic variation?

Answer 19

For a haploid number of n, the number of possible combinations is 2^n

Question 20

why does th fusion of gametes from different parents promote genetic variation

Answer 20

1) it is the start of the lift of a new individual
2) it allows alleles from two different individuals to be combined in one new individual
3) combination of alleles is unlikely to ever have existed before
4) fusion of gametes therefore promotes genetic variation in a species
5) genetic variation is essential for evolution

Question 21

What is non - disjunction the cause of?

Answer 21

Down syndrome and other chromosome abnormalities

Question 22

Why does non-disjunction happen?

Answer 22

When homologous fail to separate at anaphase- results will be either that a gamete had an extra chromosome or one less

Question 23

What do studies of age and non disjunction show?

Answer 23

That age influences chances of non- disjunction

Question 24

What is meiosis?

Answer 24

One diploid nucleus divides by meiosis to produce for haploid nuclei