3.3 Meiosis

Question 1

Meiosis is a ___________ of the nucleus to form _____________.

Answer 1

reduction division, haploid gametes

Question 2

Meiosis I overview

Answer 2

• Diploid nucleus (2n) 🡪 haploid nucleus (n)
• Homologous chromosomes separate

Question 3

Meiosis II overview

Answer 3

• Haploid nucleus (n) 🡪 haploid nucleus (n)
• Sister chromatids separate

Question 4

Product of meiosis

Answer 4

4 haploid (n) daughter cells

Question 5

Meiosis occurs in

Answer 5

GONADS (testes or ovaries)
SPERMATOGENESIS - creates sperm cells
OOGENESIS - creates egg cells

Question 6

To compensate for the chromosome doubling during fertilisation gametes undergo …

Answer 6

meiosis, which halves the chromosomes present in gametes compared to the parent

Question 7

Sexual reproduction involves ….

Answer 7

fertilisation, the fusion of gametes (sex cells), one from each parent.

Question 8

Interphase

Answer 8

In the S-phase of the interphase before meiosis begins, DNA replication takes place.

Chromosomes are replicated and these copies are attached to each other at the centromere.

The attached chromosome and its copy are known as sister chromatids.

Following S-phase, further growth and preparation take place for meiosis.

Question 9

Prophase I

Answer 9

DNA supercoils and chromosomes condense

Nuclear membrane dissolves

Centrioles migrate to the poles of the cell

The homologous chromosomes associate with each other to form bivalents (synapsis)

Chiasma = point of contact between the two non-homologous sister chromatids

Crossing-over between non-sister chromatids can take place. This results in recombination of alleles and is a source of genetic variation in gametes.

Question 10

Metaphase I

Answer 10

The bivalents line up at the equator.

Random orientation occurs - each bivalent aligns independently and hence the daughter nuclei get a different mix of chromosomes.

This is a significant source of genetic variation

Question 11

Anaphase I

Answer 11

• Spindle fibres contract.
• Homologous pairs pulled apart (separate)
• This is the reduction division – the bivalent is split and half the chromosomes move towards each pole.

Question 12

Telophase I

Answer 12

The nuclei are now haploid (N) not diploid (2N): they each contain one pair of sister chromatids for each of the species’ chromosomes.

The cytoplasm begins to divide by cytokinesis.

New nuclei form

Chromosomes decondense

n.b. If crossing-over and recombination has occurred then the sister chromatids will not be exact copies.

Question 13

Prophase II

Answer 13

Nuclear membranes dissolves

Chromosomes, consisting of two sister chromatids, condense

No crossing-over occurs

Centrioles move to opposite poles

Question 14

Metaphase II

Answer 14

Spindle fibres form and attach at the centromeres.

Pairs of sister chromatids align at the equator.

Question 15

Anaphase II

Answer 15

The sister chromatids are separated. The chromatids (now called chromosomes are pulled to opposite poles.

Spindle fibres contract and the centromeres are split.

Question 16

Telophase II

Answer 16

Nuclear membrane reforms

Chromosomes decondense

Cytokinesis begins, dividing the cells.

The end result of meiosis is four haploid gamete cells.

Fertilisation of these haploid gametes will produce a diploid zygote.

Question 17

Crossing-over between non-sister chromatids results in…

Answer 17

recombination of alleles

Question 18

Random orientation of the homologous chromosomes means…

Answer 18

there are 2n possible orientations in metaphase I. That is 223 in humans – or 8,388,068 different combinations in gametes!

Question 19

Because both crossing-over and random orientation occur during meiosis the result is…

Answer 19

effectively infinite genetic variation in the haploid gamete.

Question 20

mitosis vs meisis

Answer 20

• mitosis
  One division
  Diploid cells produced
  No crossing-over in prophase
  No chiasmata formation
  Homologous pairs do not associate and line up at the equator in metaphase
  Sister chromatids separate in anaphase
• meiosis
  Two divisions
  Haploid gametes produced
  Crossing-over in prophase I
  Chiasmata form
  Homologous pairs associate as bivalents and lined up at the equator in metaphase I
  Homologous pairs separate in anaphase I
  Sister chromatids separate in anaphase II

Question 21

For a new organism to arise sexually meiosis must occur in…

Answer 21

both parents followed by fusion of the gametes (fertilisation)

Question 22

Meiosis in a single individual produces near infinite variation, but genetic variation is further increased by:

Answer 22

Meiosis occurs in two individuals

Alleles from two organisms combine in novel ways

Question 23

benefits of genetic variation

Answer 23

Increased genetic variation produces a more resilient population that is more likely to withstand environmental change such as a disease. Genetic variation is essential for successful change by evolution.

Question 24

non-disjunction

Answer 24

homologous chromosomes fail to separate at anaphase
- Disjunction is the term used to describe the separation of chromosomes

Question 25

The result of non-disjunction is…

Answer 25

gametes that either have one chromosome too many or one too few.

Question 26

Trisomy

Answer 26

organisms with one extra chromosome (three of one chromosome instead of pair)

Question 27

Monosomy

Answer 27

organisms with one less chromosome (one chromosome instead of a pair)

Question 28

Trisomy 21

Answer 28

commonly known as Down Syndrome. The individual possesses three of chromosome 21 and 47 chromosomes in total.

Question 29

Nuchal translucency scan

Answer 29

non-invasive test, carried out between 11 and 13 weeks, an ultrasound scan looks at the nuchal fluid behind the neck of the fetus.

A large amount of nuchal fluid is strongly correlated with Down Syndrome, and the mother may be advised to have an amniocentesis or chorionic villus sample.

These tests are more invasive, but result in a karyotype of fetal cells so the parents can be sure.

Question 30

Amniocentesis

Answer 30

“amniotic fluid” & “puncture”
A sample (20 mL) of amniotic fluid is taken using a syringe.

The needle is guided by using an ultrasound.

Amniotic fluid contains cells from the fetus, and these cells are used for karyotyping.

Can be carried out in the 16th week of the pregnancy with around a 1% chance of a miscarriage

Question 31

Chorionic Villus Sampling

Answer 31

The chorion is a membrane which is a part of the placenta.

It contains many cells with villi, and some of these are taken and then cultured.

The needle is guided by using an ultrasound.

When sufficient cells have grown, a karyotype is carried out.

Can be carried out in the 11th week of the pregnancy with around a 2% chance of a miscarriage