3.3 MEIOSIS

Question 1

Meiosis

Answer 1

is a reduction division of the nucleus to form haploid gametes

the process by which sex cells(gametes) are made in the reproductive organs

Question 2

Interphase

Answer 2

In the S-phase DNA replication takes place
Chromosomes are replicated and these copies are attached to each other at the centromere (sister chromatids)

Further growth and preparation for meiosis

Question 3

Prophase I

Answer 3

Synapsis - homologous chromosomes pair up to form a bivalent

Crossing over

DNA supercoils

Nuclear membrane dissolves

Centrioles migrate to the poles of the cell

Question 4

Metaphase I

Answer 4

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

Question 5

Anaphase I

Answer 5

Spindle fibre contracts and homologous pairs are separated and pulled to opposite poles

Half the chromosomes move towards each pole

Question 6

Telophase I

Answer 6

Cytoplasm begins to divide by cytokinesis

New nuclei forms

Chromosomes decondense

Question 7

Prophase II

Answer 7

Nucelar membrane dissolves

Chromosomes condense

No crossing-over occurs

Centrioles move to opposite poles

Question 8

Metaphase II

Answer 8

Pairs of sister chromatids align at the equators

Spindle fibres form and attach at the centromeres

Question 9

Anaphase II

Answer 9

The sister chromatids are separated

Spindle fibres contract and the centromeres are split

Question 10

Telophase II

Answer 10

Four new haploid nuclei are formed

Nuclear membrane reforms

Chromosomes decondense Cytokinesis begins dividing cells

Question 11

Non-Disjunction

Answer 11

the chromosomes fail to separate correctly resulting in gametes with one extra or missing chromosome

Question 12

The failure of chromosomes to separate may occur via:

Answer 12

1. Failure to separate Anaphase I (resulting in four affected daughter cells)
2. Failure of sister chromatids in Anaphase II (resulting in only two daughter cells being affected)

Question 13

Conditions that arise from non-disjunction events include:

Answer 13

1. Patau’s Syndrome (trisomy 13)
2. Edwards Syndrome (trisomy 18)
3. Down Syndrome (trisomy 21)
4. Klinfelter Syndrome (XXY)
5. Turner’s Syndrome (monosomy X)

Question 14

Age vs Non-disjunction

Answer 14

positive strong correlation

Question 15

Karyotipying methods

Answer 15

1. Chorionic villi sampling → removing a sample of the chorionic villus (placental tissue) via a tube inserted through the cervix (11 weeks of pregnancy - 1% chances of miscarriage)
2. Amniocentesis → the extraction of a small amount of amniotic fluid (contains fetal cells) with a needle (16 weeks of pregnancy - 0.5% chances of miscarriage)