Chapter 13: Meiosis

Question 1

Gene

Answer 1

Hereditary units passed from parents to offspring that contain coded genetic information

Question 2

Locus

Answer 2

A gene’s specific location along the length of a chromosome

Question 3

Asexual reporduction

Answer 3

A single individual is the sole parent and passes copies of all its genes to its offspring without the fussion of gametes

The genomes of offspring are virtually identical of the parent’s genome

Question 4

Sexual reporduction

Answer 4

Two parents give rise to offspring that have unique combinations of genes inherited from two parents

Genetic variation is an important consequence of sexual reporduction

Question 5

Karotype

Answer 5

Image of all the metaphase chromosomes of a cell arranged in pairs starting with the longest chromosome

Question 6

Homologous chromosomes

Answer 6

Also called homologs; are identical pair of two sister chromosomes that each carry genes controlling the same set of inherited characteristics

One chromsome contains genes inherited from one parent

Question 7

Sex chromosomes

Answer 7

Two distinct chromosomes that are not identical homologs and determine the sex of an organism

XX chomosomes code for females and XY chromosomes code for males in humans

Question 8

Autosomes

Answer 8

All other chromosomes other than sex chromosomes that compose an idividuals genome

Question 9

Diploid cell

Answer 9

A cell with a two chromosome set has a diploid number of chromosomes; abbreviated as 2n

In humans the diploid number is 2n = 46 for the total number of chromosomes in somatic cells

Question 10

Haploid cell

Answer 10

A gamete cell that only contains one set of chromomes; abbreviated n

In humans the haploid number is n = 23; the set of 23 consists of 22 autosomes plus a single sex chromosome

An unfefrtililzed egg always contains an X chromosome while a sperm cell may contain an X or a Y chromosome

Question 11

Zygote

Answer 11

A fertilized egg that results from fertilization- the union of gametes culminating in the fusion of their nuclei

A zygote is thus diploid because it contains two haploid sets of chromosomes (2n) bearing genes from both the mother and the father

Question 12

Nonsister chromatids

Answer 12

A pair of one maternal and one paternal chromatid in a pair of homologous chromosomes

Question 13

Meiosis

Answer 13

Reduces the nuber of chromosome sets from diploid to haploid; each daughter cell will only have half as many chromosomes as its parent

Like mitosis, meiosis is preceeded by the duplication of chromosomes during interphase

No prometaphase in meiosis

This single duplication is followed by two consecutive cell divisions called meiosis I and meiosis II

• Meiosis I separates homologous chromsomes
• Meiosis II separates sister chromatids

Homologs appear alike however may have different versions of genes called alleles

Question 14

Prophase I

Answer 14

After interphase the sister chromatids are held together by proteins called cohesins

In early prophase I each chromosome pairs with its homolog and crossing over occurs in which nonsister chromatids are broken at precisely corresponding positions

• X-shaped regions called chiasmata are sites of crossover

Question 15

Metaphase I

Answer 15

In metaphase I, pairs of homologs line up at the metaphase plate, with one chromosome facing each pole

In humans 23 pairs of homologous chromosomes line up along the metaphase plate instead of 46

Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad- four homologous chromsomes

Question 16

Anaphase I

Answer 16

In anaphase I pairs of homologous chromosomes separate

One chromosome of each pair moves toward opposite poles, guided by the spindle apparatus

Sister chromatids remain attached at the centromere and move as one unit toward the pole

In humans 23 sister chromatids move toward each side of the cell

Question 17

Telophase I and Cytokinesis

Answer 17

In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids

Cytokinesis usually occurs simultaneously, forming two haploid daughter cells

No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated

Question 18

Prophase II

Answer 18

Question 19

Metaphase II

Answer 19

In metaphase II the sister chromatids are arranged at the metaphase plate

In humans 23 sister chromatids line up along the metaphase plate

Due to crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical

The kinetochores of sister chromatids attach to microtubules extending from opposite poles

Question 20

Anaphase II

Answer 20

In anaphase II the sister chromatids separate

The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles

In humans 23 individual chromosomes move towards each

Question 21

Telophase II and Cytokinesis

Answer 21

In telophase II the chromosomes arrive at opposite poles

Nuclei re-form and the chromosomes begin decondensing

Cytokinesis separates the cytoplasm

Marks the end of meiosis resulting in four daughter cells, each with a haploid set of unreplicated chromosomes

Each daughter cell is genetically distinct from the others and from the parent cell

Question 22

Crossing over process

Answer 22

Occurs during prophase I in meoisis

1. The two members of a homologous pair associate loosely along their lengths
2. The DNA of one paternal and one maternal non-sister chromatid align so that their genes correspond percisely with one another
3. A zipper-like complex called the synaptonemal complex forms that holds homologous pairs tightly together
4. The DNA of the non-sister chromatids are broken at percisely corresponding points
5. During this association called synapsis the DNA breaks are re-joined so that the two non-sister chromatids each now contain DNA from the other
6. After crossing over occurs the synaptonemal complex disassembles and the homologs move away from each other slightly forming a visible chiasmata where the crossing over occured
7. The homologs remain attached because sister chromatid cohesion still holds them together although some DNA is no longer attached to its original chromosome

Question 23

Unique differences between meiosis and mitosis

Answer 23

1. Synapsis and crossing over
2. In metaphase I homologous pairs line up at the metaphase plate rather than individual chromosomes
3. In anaphase 1 homologous pairs are separated while the sister chromatids remain attached; sister chromatid cohesion is released in anaphase II

Question 24

Origins of genetic variation among offspring

Answer 24

1. Independent assortment of chromosomes
2. Crossing over
3. Random fertilization

Question 25

Independent assortment of chromosomes

Answer 25

Each pair of chromosomes sorts maternal and paternal homologs into daughter cells independently of the other pairs

Each daughter cell thus represents one possible outcome among all possible combinations of maternal and paternal chromsomes

The number of combinations possible when chromosomes assort independently into gametes is 2ⁿ, where n is the haploid number

In humans n = 23 so there are about 8.4 million (2²³) possible combinations of chromosomes

Question 26

Crossing over outcome

Answer 26

Crossing over produces recombinant chromosomes which combine DNA inherited from each parent and results in novel chromosomes

Contributes to genetic variation by combining DNA from two parents into a single chromosome

In humans an average of one to three crossover events occurs per chromosome

Question 27

Random fertilization

Answer 27

Random fertilization adds to genetic variation because any sperm can fuse with any ovum- unfertilized egg

In humans the fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations