4 - Mitosis/Meiosis/Recombination/Mendel's Laws

Question 1

Define karyotype

Answer 1

Morphological characteristics of a cell’s chromosomes.

22 homologous pairs (46 total chrom) of autosomal chromosomes plus one pair of “sex determination” chromosomes.

23 homologous pairs = diploid (2N=46).

Question 2

Define haploid

Answer 2

N=23, the number of chrom from each parent.

Members of a pair are homologous to each other.

Question 3

Define genotype

Answer 3

Person’s biochemical genetic makeup.

Mediated by differences between allele within a homologous pair

Question 4

Define phenotype

Answer 4

Biochemical, anatomic, physiological or behavioral TRAIT.

Mediated by environment and genotype.

Expression of genes into the protein they encode confers phenotype.

Question 5

What is an allele?

Answer 5

The genome has two homologous copies of each gene, one from each member of a homologous chromosome pair.

Each of these two copies is termed an allele.

Can be dom or recessive

Question 6

What are examples of homozygous recessive diseases?

Answer 6

Cystic fibrosis, albinism, PKU, and tay-sachs.

Question 7

What occurs after S phases with DNA?

Answer 7

DNA content is 4N rather than 2N.

Mitosis occurs: goal is to restore 2N DNA precisely. This is activated in late G2.

Question 8

What activates M-phase (mitosis)?

Answer 8

cdc25 phosphatase dephosphorylates CDK1 -> nucleus.

Cyclin B/CDK1 phosphorylates nuclear targets such as:

• centrosome causes mitotic spindle activation
• lamins which cause nuc envelope to breakdown

Question 9

What are the basic phases of Mitosis?

Answer 9

P: CyclinB/CDK1 > condensins > chromatic condensation

M: chromatids allign at equator

A: APC degrades cohesion then chromatids move

T: daughter nuclei form, mediated by lamin B

C: 2 daughter cells formed

Question 10

What occurs in early prophase?

Answer 10

CyclinB/CDK1 phosphorylate condensin and centrosomes move to spindle poles

Question 11

What occurs in late prophase? What are the types of mts involved?

Answer 11

CyclinB/CDK1 phosphorylates things causing the nuc envelope to breakdown.

3 types of mts:

1. astral cause state shaped centrosome
2. kinetochore: attach to midpoint of chrom
3. polar: control movement of chromosomes in M-phase

Question 12

What occurs during metaphase?

Answer 12

Sister chromatids are at equator attached to kinetochore mts.

(Note: homologous chromosomes do NOT pair up in mitosis, this occurs in meiosis)

Question 13

What occurs in chemotherapy with metaphase?

Answer 13

Vinblastine inhibits mt assembly by binding tubulin.

Taxol inhibits mt dis-assembly

Both block mitosis.

Question 14

What initiates anaphase?

What occurs in anaphase?

Answer 14

Initiated by anaphase-promoting complex (APC).

In kinetochore, APC degrades securin to release separase, which degrades cohesin. Result is release of sister chromatids.

Question 15

What mechanism do sister chromatids use to move to opposite poles?

Answer 15

Push-pull mechanism.

Push: elongation of polar mts
Pull: shortening of kinetochore mts

Question 16

What occurs during telophase? What is this called?

Answer 16

Kinetochore tubulrs are extinct and polar tubules are still elongating.

Nuclear envelopes reform via lamin B.

Called karyokinesis (nuclear division).

Question 17

What occurs during cytokinesis?

Answer 17

Cleavage furrow, cells separate by pinching off.

Mediated by actin and myosin.

Question 18

How does meiosis differ from mitosis

Answer 18

In meiosis there are two divisions resulting in 4 cells. Daughter cell genotypes are variable. Chromosome number reduced to haploid.

There is no S phase in meiosis 2.

There’s pairing of homologous chrom at prophase 1.

Centromeres divide only at anaphase 2.

Question 19

When does crossing over occur in meiosis? When do homologous chromosomes separate in meiosis? When do sister chromatids separate?

Answer 19

Crossing over occurs in prophase 1, therefore there are only 23, not 46 positions occupied on the equitorial plate.

Homologous chrom separate in anaphase 1.

Sister chromatids separate in anaphase II.

Question 20

Why is meiosis 1 called a reduction division?

Answer 20

Chromosome number in the two daughter cells is reduced by half.

This is because the homologous chromosomes (each having 2 sister chromatids) split during anaphase 1.

Question 21

Why is meiosis called a mitotic division (without S phase)?

Answer 21

The centrosomes (sister chromatids) finally divide at anaphase 11 and are portioned into the daughter cells.

Result being that there are four haploid cells.

Question 22

Describe the genotypes of the daughter cells from mitosis?

Answer 22

They remain identical.

Question 23

Describe the genotypes of gametes from meiosis?

Answer 23

It exploits heterozygosity, resulting in gametes with different genotypes.

Question 24

What does Mendel’s first law: the law of segregation state?

Answer 24

From meiosis:

Each gamete gets one allele or the other (never both) from the parental cell.

Each allele is segregated into a separate gamete.

Question 25

What does Mendel’s second law: the “independent inheritance” law state? Where does this occur?

Answer 25

Alleles that encode different traits assort independently of each other.

Exp. inheritance of stature and eye color can’t be linked, because each trait is inherited independently

Independent assortment occurs at meiosis 1 when homologous chrom go to separate cells.

Question 26

Due to alleles for traits assorting independently, what is the phenotypic ratio that always occurs? What about for the F2 progeny?

Answer 26

3:1

F2 progeny - 9:3:3:1 ratio

Question 27

What are two important points during meiosis 1?

Answer 27

Crossing over - occurs during prophase 1

Independent assortment: occurs at anaphase 1

Question 28

Describe crossing over in prophase one of meiosis?

Answer 28

Genetic recombination that occurs via non-sister chromatids of paired homologous chromosomes.

Mediated by dsDNA breaks between regions of homology.

Question 29

What is the point where crossing over occurs called?

Answer 29

The chiasmata, which is thought to be random.

Question 30

How does crossing over indicate gene organization on chromosomes?

Answer 30

As distance between two genes on a chromosome increases, the probability increases that cross-over will occur.

Genes have a high crossover freq between that are said to be loosely linked. Low crossover freq are tightly linked (closer together).

Question 31

What does 1% frequency of crossover equal in terms of distance from one gene to another? What does this mean in humans?

Answer 31

1% freq of crossover = 1 centiMorgan (cM)

In humans, 1 cM = 1 million bp DNA.