Lecture 9B

Question 1

when is the Crossing over b/w homologous chromosomes

Answer 1

prophase I

Question 2

Metaphase I?

Answer 2

Homologous chromosomes line up at metaphase plate

Random line up of replicated, condensed homologous pairs (in tetrads) at metaphase plate

Question 3

Anaphase I?

Answer 3

Homologous chromosomes are separated

Question 4

Telophase I?

Answer 4

Nuclear membrane reforms: 2 non-identical haploid nuclei are formed.
Meiosis I complete

Question 5

Prophase I?

Answer 5

Replicated chromosomes condense (1 replicated chromosome made up of 2 identical sister chromatids)

Spindle fibers appear

Question 6

what is a tetrad?

Answer 6

in prophase 1 Replicated homologous chromosomes pair up to form a tetrad

Question 7

Prometaphase I?

Answer 7

Nuclear membrane breaks down
Spindle fibers attach to replicated condensed chromosomes
Tetrads still present

Question 8

Independent assortment of chromosomes:

Answer 8

homologous pairs line up in random orientations at the metaphase plate in preparation of anaphase I, where they will be separate

Question 9

how do we know the # of possible combinations?

Answer 9

2n where n=haploid number

Question 10

Prometaphase II?

Answer 10

nuclear envelope breaks down, spindle fibers attach to condensed replicated chromosomes

Question 11

Metaphase II?

Answer 11

condensed replicated chromosomes line up

Question 12

Anaphase II?

Answer 12

sister chromatids separated → unreplicated condensed chromosomes

Question 13

Telophase II?

Answer 13

nuclear envelope reforms, unreplicated chromosomes decondense,
4 haploid nuclei in 2 cells

Question 14

why does Each haploid cell has a different combination of genes

Answer 14

1. Maternal & paternal chromosomes of homologous pairs separate independently (independent assortment)
2. Crossing-over: DNA segments exchanged b/w maternal & paternal homologues

Question 15

nondisjunction?

Answer 15

Nondisjunction is the failure of the chromosomes to separate, which produces daughter cells with abnormal numbers of chromosomes

Question 16

Nondisjunction in meiosis 1 vs meiosis 2?

Answer 16

Nondisjunction in meiosis 1: 4 cells with abnormal # chrom.

Nondisjunction in meiosis 2: 2 cells with abnormal # chrom.

Question 17

Aneuploidy?

Answer 17

Have 1 extra chromosome : 2n + 1
or
Missing 1 chromosome: 2n - 1

Usually result in prenatal death (miscarriage)
More common than polyploidy

Cause: nondisjunction

Question 18

Polyploidy?

Answer 18

Multiple SETS of chromosomes (3n, 4n, etc)
Common in plants, rare in animals (miscarriage)
Fertilization of egg by >1 sperm