Session 5: Mitosis & Meiosis

Question 1

How many alleles of a specific gene are found in the G2 phase of the cell cycle? Why?

Answer 1

2, each gene is inherited in 2 alleles because each comes from one parent so there would be 2 different alleles for a trait.

Question 2

Why do humans have 2 copies of each chromosome?

Answer 2

Humans are diploid so each cell carries 2 copies of each chromosome, one from mother and one from father

Question 3

What are the phases of the cell cycle? What occurs in each phase?

Answer 3

G0 - Cell is performing its function without actively preparing to divide - rest
G1 - cell grows
S - DNA replication
G2 - Cell prepares for cell division, more growth & organelles and proteins develop
M - Cell division (mitosis or meiosis)

Question 4

How many DNA molecules are there in one chromosome after G2 phase?

Answer 4

2 DNA molecules (each chromatid consists of 1 DNA molecule)

Question 5

What are the main cell cycle checkpoints & what occurs at each?

Answer 5

G1 - checks for nutrients, growth factors & DNA damage
G2 - Check for cell size & DNA replication
Metaphase - check for spindle attachment

Question 6

What are telomeres? What is their function? How would they be identified under a telescope?

Answer 6

They are regions of repetitive DNA found at either end of chromosomes, they appear as white dots under a microscope.
Telomeres protect the end of chromosomes during replication, making sure all genes on chromosomes are copied and no info is lost during replication

Question 7

What is the function of centromeres?

Answer 7

Sequence that links sister chromatids together

Question 8

What is G-banding and what do they tell us about chromosomes?

Answer 8

Chromosome banding = thin alternating light & dark regions along length of chromosome
Enzymatic digestion followed by Giemsa stain
- Identify translocations, duplications & loss of specific chromosomes

Question 9

How is chromosome painting used to identify chromosomes?

Answer 9

Fluorescent markers label different parts of chromosomes

Question 10

What is an allele?

Answer 10

A variant of a gene

Question 11

At which stage of mitosis are sister chromatids moved to opposite poles?

Answer 11

Anaphase

Question 12

At which stage of mitosis do chromosome decondense, spindle fibres disappear and nuclear membrane reforms?

Answer 12

Telophase

Question 13

At which stage of mitosis does the nuclear membrane break down, spindle fibres appear and chromosomes appear?

Answer 13

Prophase

Question 14

At which stage of mitosis do chromosomes align and condense at the metaphase plate?

Answer 14

Metaphase

Question 15

What does mitosis produce after cytokinesis?

Answer 15

2 identical diploid daughter cells are produced with the same chromosome content as parent

Question 16

What type of cells does mitosis occur in?

Answer 16

Somatic cells

Question 17

What type of cells does meiosis occur in?

Answer 17

Germ cells

Question 18

What does meiosis produce after cytokinesis?

Answer 18

4 non-identical haploid cells with 1/2 the number of chromosomes in each cell

Question 19

How do sex chromosomes find each other

Answer 19

They share similar sequence

Question 20

During which stage of meiosis are homologous chromosomes paired up?

Answer 20

Prophase 1

Question 21

What are the implications of recombination?

Answer 21

Genetic variation

Question 22

What is spermatogenesis?

Answer 22

a meiotic process producing sperm

Question 23

What is oogenesis?

Answer 23

a meiotic process producing oocytes

Question 24

Compare oogenesis & spermatogenesis.

Answer 24

• 1 oocyte (2n) gives 1 egg (n) & 3 polar bodies whilst 1 spermatocyte (2n) gives 1 egg (n) and 3 polar bodies
• Length of oogenesis = 12-50 years whilst length of spermatogenesis = approx 60 days
• Oogenesis = non-motile gametes whilst spermatogenesis = motile games

Question 25

What is aneuploidy? What can cause it?

Answer 25

condition of having an abnormal number of chromosomes in a haploid organism
can be caused by non-disjunction during meiosis or miosis

Question 26

What is non-disjunction?

Answer 26

A failure to separate pairs of chromosomes lined up at the metaphase plate = produces daughter cells with abnormal number of chromosomes

Question 27

What is trisomy? What is an example of a condition caused by meiotic trisomy?

Answer 27

3 copies of a chromosome instead of 2.
Eg Down’s syndrome = 3 copies of chromosome 21 instead of 2

Question 28

What is monosomy? What is an example of a condition caused by miotic trisomy?

Answer 28

Only 1 copy of chromosome rather than 2 - missing chromosome.
Eg Turner’s syndrome = only 1 X chromosome, missing chromosome

Question 29

What is the difference between meiosis 1 and meiosis 2?

Answer 29

meiosis 2 is cell division without DNA replication

Question 30

When and how do homologous chromosomes pair up in meiosis 1?

Answer 30

During prophase 1
- Homologous chromosomes pair up forming bivalents
- Crossing over occurs at chiasmata between maternal and paternal copy
- Recombinant chromosomes are formed