3.3 Meiosis

Question 1

What kind of human cells are made in meiosis?

Answer 1

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

Question 2

Meosis starts with one diploid nucleus being divided to produce

Answer 2

Four genetically distinct haploid nuclei

Question 3

What happens to the number of chromosomes in the 1st cellular division in meiosis

Answer 3

The first meiotic division separates pairs of replicated homologous chromosomes (X X) to halve the chromosome number (diploid → haploid)
(X) (X)

46 chromosomes (92 chromatids) → 23 chromosomes (46 chromatids)

Question 4

What happens to the number of chromosomes in the 2nd cellular division in meiosis

Answer 4

The second meiotic division separates sister chromatids from the homologous pair of replicated chromosomes that were separated during meiosis I
(X) (X)
Results in four haploid daughter cells:
(I) (I) (I) (I)

23 chromosomes (46 chromatids) → 23 chromosomes (23 chromatids)

Question 5

Stages of meiosis 1 and meiosis 2

Answer 5

Meiosis 1:
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I

Meiosis 2:
- Prophase II
- Metaphase II
- Anaphase II
- Telophase II

Question 6

What type of cell do we begin with before meiosis?

Answer 6

A diploid cell (I I)

Question 7

What happens during the S phase in interphase?

Answer 7

DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.
The chromosomes replicate and now each have sister chromatids.

46 chromosomes → 46 chromosomes (92) chromatids

Question 8

Prophase I

Meiosis I

Answer 8

• Chromosomes condense
• Nuclear membrane dissolves
• Spindle fibres form
• Homologous chromosomes pair up and form a bivalent (or tetrad) through a process called synapsis. The homologous chromosomes are held together at points called chiasmata
• Crossing over occurs (between non-sister chromatids at these chiasmata)

Question 9

What is crossing over and what are its consequences?

Answer 9

• When the chromosomes undergo synapsis and pair up to form a bivalent, they are held together at points called chiasmata
• At these points, crossing over occurs, which refers to the exchange of non genetic material (between non-sister chromatids)
• This means that the sister chromatids will no longer be identical
• Results in recombinant chromosomes
• Promotes genetic variation

Question 10

Metaphase I

Answer 10

• Homologous pairs of chromosomes align along the equator of the cell
• The spindle fibres connect to the homologous pairs at their centromeres
• There is random ortientation/independent assortment, meaning that each chromosome may face either pole of the cell. This means different combinations of maternal and paternal chromosomes can be inherited when the chromosomes separate in anaphase.

Question 11

Random orientation/independent assortment

Answer 11

• Occurs during methaphase I
• Random ortientation/independent assortment means that each chromosome may face either pole of the cell.
• This means different combinations of maternal and paternal chromosomes can be inherited when the chromosomes separate in anaphase.
• Leads to genetic variation

Question 12

Anaphase I

Answer 12

• Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell

Note: the sister chromatids remain attached at their centromeres.

Question 13

Telophase I

Answer 13

• Nuclear envelopes reassemble.
• Spindle fibres disappear.
• Cytokinesis divides cell into two.
• Results in two haploid daughter cells

Question 14

Prophase II

Answer 14

• Nuclear envelope breaks down
• Spindle forms.
• Centrosomes move to opposite poles

Question 15

Metaphase II

Answer 15

• Chromosomes align along equator of cell.
• Spindle fibres attarch to the chromosomes at their centromere

Question 16

Anaphase II

Answer 16

• Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles

Question 17

Telophase II

Answer 17

• Nuclear membrane reforms
• Chromosomes decondense
• Spindle fibres dissapear
• Cells divide (cytokinesis) to form four haploid daughter cells

Question 18

What does meiosis allow on sexually reproducing organisms?

Answer 18

Meiosis allows sexually reproducing organisms, which have diploid cells, to halve their chromosome numbers (to produce gametes which are haploid cells) and allow a sexual life cycle to occur.

Question 19

The fusion of two haploid gametes results in…

Answer 19

the formation of a diploid zygote.
- The production of for distinct gamete cells allows genetic variation when the fusion of sperm and eggs occurs to create a diploid zygote cell.
- This zygote can then divide by mitosis and differentiate to form a developing embryo.

Question 20

Non-disjunction

Answer 20

When chromosomes fail to separate.
- This can result in gametes with one extra or one missing chromosome

Question 21

What causes Down syndrome

Answer 21

Down syndromw is caused by non-disjunction and happens when a zygote receives an extra copy of chromosome number 1.

Question 22

Relationship between age of parents and chances of non-disjunction

Answer 22

Studies have shown that as the age of parents increases, the chances of non-disjunction increases.

Question 23

Methods used to obtain cells for karyotype analysis – e.g. chorionic villi sampling and amniocentesis + the associated risks

Application

Answer 23

Karyotyping is the process by which chromosomes are organised and visualised for inspection. Karyotyping is typically used to determine the gender of an unborn child and test for chromosomal abnormalities.
Fetal cells can be obtained so karotyping can take place. There are two ways of obtaining these cells:
1. Amniocentisis: a needle is inserted through the abdomen and into the amniotic sac, amniotic fluid is withdrawn which contains the fetal cells.
- This procedure can be done later in pregnancy with a low risk of miscarriage
2. Chorionic Villi Sampling: a tube is inserted through the cervix and is used to remove part of the placenta called called the chorionic villi which contains the fetal cells.
- This procedure can be done earlier in pregnancy but runs a greater risk of miscarriage