Chapter 2 - Mitosis & Meiosis

Question 1

PAGES 55-75 (1.2)

Meiosis?
Mitosis?

Answer 1

Mechanisms by which cells distribute the genetic info contained in their chromosomes to progeny cells in a precise, orderly fashion.

Question 2

1. If an organism has a diploid number of 16, how many chromatids are visible at the end of mitotic prophase?
2. HOw many chromosomes are moving to each pole during anaphase of mitosis?

Answer 2

1. 32 chromatids would be visible at the end of prophase. Only at this end would I be able to see with a light microscope, that each chromosome consists of 2 parts (called sister chromatids). They are considered chromosomes, even though they are two chromatids strapped together.
2. The two strapped-together chromatids are pulled apart – so 16 to each end. Still called chromosomes at this time, even though the single strands are single chromatids.

Question 3

How do we know that chromosomes exist in homologous pairs?

Answer 3

Textbook: The study of somatic cells tells us that the chromosomes exist as homologous pairs all the time. By taking any cell from any part of the body (not sex cells), and placing it under the microscope, we can see they all contain the same amount of chromosomes. 23 from mom and 23 from dad. So then Chromosome 1 from mom can be matched up with Chromosome 1 from dad and all the alleles will match up too.

Question 4

How do we know that DNA replication occurs during interphase, not early in mitosis?

Answer 4

Because when the nucleolus disentegrates in prophase, the loose chromatin has condensed and it’s already in paired sister chromatids.

Textbook: The initiation and completion of DNA replication can be detected by incorporation of labelled precursors into DNA. This would show that the DNA content in G2 would be 2x that of G1 phase.

Question 5

An organism has a diploid # of 16 in a primary oocyte:

1. How many tetrads are present in the 1st meiotic prophase?
2. How many dyads are present in the 2nd meiotic prophase?
3. How many monads migrate to each pole during the 2nd meiotic anaphase?

Answer 5

1. This is where ‘crossing over’ takes place. Each tetrad consists of 2 pairs of sister chromatids, joined at the centromere. So if a diploid has # of 16 chromosomes, there will be 8 tetrads in prophase 1 of meiosis.
2. In the 2nd stage of meiosis, there’s only one set of chromosomes. # of dyads = haploid #.
   Ex: diploid # is 16 chromosomes (haploid is 8 then) = 8 dyads.
3. Each chromosome pulled to the poles is considered a monad. So if diploid is 16, then 8 monads move to each pole of the cell.

Question 6

What are the consequences of mitosis or meiosis not executing with absolute fidelity?

(total and complete)

Answer 6

Question 7

How do these contribute to storage, expression, or transmission of genetic info?
1. Chromatin
2. Nucleolus
3. Ribosome
4. Mitochondrion
5. Centriole
6. Centromere

Answer 7

1. To efficiently package DNA into a small volume to fit inside the nucleus. Packing DNA into chromatin fibres ensures the DNA doesn’t break and stays safe.
2. Site of ribosome biosynthesis and assembly
3. Where proteins are made by translating mRNA (the site)
4. Involved making the energy for the cell
5. cytoplasmic structures involved in migration of chromosomes during mitosis AND meiosis
6. point of attachment for the sister chromatids and where spindle fibres attach to chromosomes.
   7.

Question 8

Define these clearly:
1. Diploidy
2. Homologous chromosomes
3. Haploidy

Answer 8

1. When cells contain homologous chromosomes and denoted by 2n. For humans, it’s somatic cells that have this and they have 46 chromosomes (23 pairs of homologous chromosomes)
2. When there are sister chromatids and each chromatid is the same
3. When the cell has a single set of chromosomes and denoted by 1n. For humans, it’s gamete cells that have this and they only have 23 chromosomes.

Question 9

If 2 chromosomes of a species are the same length and have similar centromere placements and yet are not homologous:

What is different about them?

Answer 9

• Different gene position, so they can’t be paired up
• Different banding pattern

Question 10

Each stage of Mitosis.

Answer 10

Question 11

How are chromosomes named on the basis of their centromeres?

Answer 11

Question 12

Telophase: contrast this stage in mitosis for plants & animals.

Answer 12

1. IN ANIMAL CELLS: Cytokinesis occurs when the outer cell membrane starts to get constricted (cell furrow) to start the end stage.
2. IN PLANT CELLS: the cell plate forms in the middle

Question 13

The cell cycle. Describe.

Answer 13

Question 14

VOCAB:

1. Synapsis.
2. Bivalents.
3. Chiasmata.
4. Crossing over.
5. Chromomeres.
6. Sister Chromatids.
7. Tetrads.
8. Dyads.
9. Monads.

Answer 14

1. Synapsis : in meiosis, the synapse refers to the point-by-point homologous chromosomes pairing during the prophase 1 stage. Synapsis allows the ‘crossing-over’ to happen.
2. Bivalents: each synapsed pair of 2 identical chromosomes is initially called this bivalent (equal to the haploid #)
3. Chiasmata : visible manifestations of crossing-over between non-sister chromatids.
4. Crossing Over: the physical process of chromosomal material exchange (between the homologous chromosomes). ONLY occurs in Metaphase 1 of Meiosis 1)
5. Chromomeres : bands on polytene chromosomes, due to thickness of replicated chromosomes

Question 15

1. Genetic content of sister vs. non-sister chromatids (during early Prophase 1)?
2. How does it change when tetrads have aligned during metaphase1?

Answer 15

#2 : Once the tetrads have aligned in the middle of the cell, the spindle fibres divide the sister chromatids and move the separated chromatids towards opposite poles.

#1:

Question 16

Consider the end results of meiosis and mitosis: why do homologs need to pair for meiosis, but not for mitosis (not desirable)?

Answer 16

In meiosis, the homologs need to pair so that the # of chromosomes can be halved to the haploid #.

In Mitosis though, 2 diploid daughter cells are made from 1 diploid parent. At metaphase, the chromosomes need to be unpaired. This allows the centromere to be split apart and chromosomes taken to either pole of the cell.

Question 17

Spermatogenesis vs. Oogenesis?

Why do polar bodies form?

Answer 17

The main fxn of polar bodies involves discarding excess chromosomes as well as chromatids to make a healthy haploid egg cell.

Question 18

Why does genetic variation occur with meiosis, but not mitosis?

Answer 18

Genetic variation occurs due to ‘crossing over’ in the Metaphase 1 stage of Meiosis. ONLY in meiosis. This involves the physical interchange of chromosomal segments between the maternal and paternal chromatids. It creates new allele combo’s.

In Mitosis, this never happens. No crossing over, so no such thing as genetic variation.

Question 19

A diploid cell contains 3 pairs of homologous chromosomes designated C1 & C2, M1 & M2, AND S1 & S2.

No crossing over occurs.

What combos are possible in:
1. Daughter cells following mitosis?
2. Cells undergoing the 1st meiotic metaphase?
3. Haploid cells following both divisions of meiosis?

4. Predict the # of different haploid cells that could be produced by meiosis if a fourth chromosome pair (W1 & W2) were added.

Answer 19

1. Since 2 daughter cells are made from 1 parent cell, the only combo’s made are C1, C2, M1, M2, S1, S2.
2. In Metaphase 1, the homologous pairs of chromosomes are split into 2 different cells, now haploid. 8 possible combo’s.
3. A daugher cell could get:
   C1/M1/S1 -
   C1/M1/S2-
   C2/M1/S1-
   C2/M2/S1-
   C2/M2/S2-
   C2/M1/S2-
   C1/M2/S1-
   C1/M2/S2-
4. 2^4 = 16 Combinations

Question 20

During oogenesis in an animal species with a haploid # of 6, one dyad undergoes non-disjunction during meiosis II.

Following the 2nd meiotic division, this dyad ends up intact in the ovum. How many chromosomes are present in:
1. the mature ovum?
2. the 2nd polar body?
3. Post fertilization, what chromosome condition is created?

Answer 20

1. Mature ovum = 7 chromosomes
2. in Oogenesis, one cell has more cytoplasm and the other has less cytoplasm. Since the dyad is not divided properly, thus ends up intact in the ovum at the end of the cellular division. In the 2nd polar polar body, it reduces by 1, so it’s 5.
3. If a normal sperm fertilizes the ovum containing the dyad, then the embryo will have 13 chromosomes. (12 + 1)

Question 21

What is the probability with an organism of haploid # of 10, a sperm will be formed that contains all 10 chromosomes whose centromeres were derived from maternal homologs?

Answer 21

In any organism that performs sexua reproduction, gametogenesis forms cells that can have chromosomal combination originating from mum or dad or a combo.

In every sexually reproducing organism, each tetrad has half of the maternal homolog for each tetrad. So the probability of formation of a sperm containing all the chromosomes derived from maternal homologs can be calculated as shown below:

Question 22

The nuclear DNA content of a single sperm cell inDrosophila m. is approx. 0.18 picograms.

What would be the expected nuclear DNA content of a primary spermatocyte in Drosophilia?

What would be the expected nuclear DNA content of a somatic cell ( non-sex cell) in the G1 phase?

What would be the expected nuclear DNA content of a somatic cell at metaphase?

Answer 22

1. In spermatogenesis, 1 spermatocyte froms 4 sperms, so it would be 0.18 pcg x 4 = 0.72 pcg
2. In a somatic cell, it would be doubled so 2 x 0.18 pcg = 0.36 pcg
3. At metaphase in a regular, somatic cell: if the DNA is already doubled in prep for 2 daughter cells to be made, it would have to be (0.18 x 2) x 2 at metaphase = 0.72 pcg. —-which would immediately be brought down to 2 daughter cells with 0.36 pcg in each.

Question 23

Chromatic fibre vs. Mitotic Chromosome.

How are they contrasted?
How are they related?

Answer 23

Question 24

Describe the ‘folded-fibre’ model of the mitotic chromosome

Answer 24

The folded fibre model describes the conversion and condensation of chromatin fibres into chromosomes.

Question 25

You have a slide of an unknown organism. It has 12 chromosomes. Two that are clearly smaller than the rest appear identical in length and centromere placement.

Describe all that you can about these chromosomes.

Answer 25

Can be the sex chromosomes. The other ten would be the autosomal chromosomes. During metaphase, all 12 would be lined up at the middle with spindle fibres attached to the kinetechores.

Question 26

If you study 50 primary oocyctes in an animal through oogenesis:
1. How many secondary oocytes would be formed?
2. How many first polar bodies would be formed?
3. how many ootids would be formed?

If you study 50 primary spermatocytes in an anima through the stages of spermatogenesis:
1. how many secondary spermatocytes would be formed?
2. How many spermatids would be formed?