Ch. 2 - Basic Processes

Question 1

What are homologous chromosomes?

Answer 1

• -> are similar, but not identical
• -> carry genes for same inherited characteristic
• -> may carry different version of same gene
• -> one is inherited from mother, other fromf ather

Question 2

During G1, each chromosome of a homologous pair contains how many chromatin?

Answer 2

1

Question 3

What are sex chromosomes?

Answer 3

sex determining chromosomes = not homologous, but behave as homologs during meiosis b/c pseudoautosomal region contain homologous sequences that are able to pair and recombine during meiosis.

Question 4

What are centromeres and where are they located?

Answer 4

constricted regions on chromosomes

Question 5

What is mitosis?

Answer 5

–> it partitions chromosomes into dividing cells.

–> produces daughter cells with a full diploid complement of chromosomes

Question 6

What is karyokinesis?

Answer 6

genetic material partitioned to daughter cells during nuclear division

Question 7

what is cytokinesis?

Answer 7

cytoplasmic division follows

Question 8

What is the cell cycle composed of?

Answer 8

interphase and mitosis.

Question 9

What does interphase include?

Answer 9

• -> S phase: DNA is synthesized

- -> two gap phases (G1 and G2)

Question 10

What happens in G0 stage?

Answer 10

a stage in G1 phase where cells become nondividing, but maintain a metabolically active state
–> some cell types are permanently in G0 while others can re-enter the cell cycle under certain circumstances

Question 11

What happens in metaphase?

Answer 11

• -> kinetochore: proteins associated with centromere
• -> spindle fibers: bind to kinetochore and facilitate chromatid migration to opposite poles
• -> cohesin: nprotein complex that holds sister chromatids together
• -> separase: enzyme that degrades cohesin
• -> shugoshin: protein that protects cohesin from being degraded by separase

Question 12

What happens in anaphase?

Answer 12

Shugoshin is degraded, therefore separase is able to degrade cohesion at the centromeres.

Disjunction: sister chromatids separate and migrate to opposite poles

Separated sister chromatids are called ‘daughter chromosomes’

Question 13

What are the cell cycle checkpoints?

Answer 13

G1/S checkpoint

• -> monitors size cell has achived
• -> evaluates condition of DNA

G2/M checkpoint

• -> monitors if DNA replication is incomplete
• -> monitors damaged DNA

M checkpoint
–> monitors successful formation of spindle fiber system and attachment to kinetochores

Question 14

What is meiosis?

Answer 14

• -> reduces amount of genetic material by half

- -> produces haploid gametes or spores, each containing one chromatid of a homologous pair of chromosomes

Question 15

What are meiosis I and meiosis II?

Answer 15

meiosis I: reductional division

meiosis II: equational division

DNA synthesis occurs during interphase before the beginning of meiosis I and does NOT occur again before meiosis II

Question 16

Meiotic prophase I: Leptonema

Answer 16

chromosomes appear as long, single threads, unassociated with each other

Question 17

Meiotic prophase I: Zygonema

Answer 17

formation of synaptonemal complex: synapsis

each pair of homologous chromosomes is known as bivalent

Question 18

Meiotic prophase I: Pachynema

Answer 18

each bivalent becomes shorter, thicker, and two sister chromatids split and become visible: 4 membered structure (2 homologs each containing two sister chromatids is called a ‘tetrad’)

Question 19

Meiotic prophase I: ‘crossing over’

Answer 19

excahnge of genetic material b/w nonsister chromatids

Question 20

Meiotic prophase I: Diplonema

Answer 20

within tetrads, sister chromatids separate ‘Chiasma’: Where chromatids remain intertwined

Question 21

Meiotic prophase I: Diakinesis

Answer 21

• -> nucleus and nuclear envelope break down
• -> centromeres attach to spindle fibers (both sister chromatids of a homologous pair pointing to same pole; two homologs pointing to opposite poles)

Question 22

What happens during metaphase I?

Answer 22

tetrads align in random orientation at the metaphase plate

Question 23

What happens during anaphase I?

Answer 23

homologous chromosomes separate and move toward the poles (dysjunction).

the sister chromatids remain attached at centromeres (cohesion protected by shugoshin) and are called dyads.

Question 24

What happens in telophase I?

Answer 24

each pole now has a haploid set of chromosomes (each containing 2 sister chromatids)

Question 25

How many haploid daughter cells does cytokinesis yield?

Answer 25

it yields two haploid daughter cells.

nuclear envelope may form around chromosomes in some species

Question 26

What happens in second meiotic division?

Answer 26

dyads align at metaphase plate.

shugoshin is degrade and this allows sister chromatids to separate towards opposite poles (anaphase II).

following telophase II and cytokinesis, each haploid daughter cell from meiosis II has one member (a monad) from each pair of homologous chromosomes.

Question 27

When does replication occur?

Answer 27

prior to mitosis and meiosis.

Question 28

Different eukaryotic organisms have different life cycles that involve ____ phases.

Answer 28

alternation of haploid and diploid phases.

Question 29

Why in a diploid species you cannot directly see the results of meiosis?

And what is one usful method to use in order to see the prodcuts for meiosis?

Answer 29

because the gametes do not have a phenotype.

therefore, in order to look at the products of meiosis you need to cross an organism with another organism.
–> ex: you may carry out a self cross, a testcross, or a backcross

however, fungi spend most of their life cycles as haploid organisms and if the spore has a detectable phenotype then you can deduce its genotype and therefore see the product of meiosis directly. This is known as ‘Tetrad Analysis’

Question 30

What is tetrad analysis? Give an example.

Answer 30

it’s when you are deducing the genotype of an organism to see the product of meiosis directly when the spore has a detectable phenotype.

Ex1: Saccharomyces cerevisiae

Ex2: Neurospora crassa / Sordaria fimocola
***these organisms produce asci containing ‘ordered’ tetrads