Chromosomal Inheritance

Question 1

Body Cells

Answer 1

have 46 chromosomes in 23 pairs #1- 22 in pairs + XX (female) or XY (male) are called diploid (2N)

Question 2

Sex Cells

Answer 2

(gametes – sperm and eggs) that have only 23 chromosomes (#1-22 not in pairs, plus X or Y), are called haploid (N)

Question 3

Karyotype

Answer 3

Sampling of fetal chromosomes

• Obtain fetal body cell sample • Stimulate cells to divide in
tissue culture
• Stop cell division when chromosomes are most compact and easily seen
• Apply stain
• Examine microscopically

Question 4

Genes

Answer 4

Units of heredity on chromosomes that completely control our cells (and all of us)

Question 5

The Cell Cycle

Answer 5

2 parts

1. Interphase
   - G1 stage: cell doubles in organelles, growth in size
   - S Stage: DNA replication occurs as 2 sister chromatids form, connected by a centromere
   - G2 stage: proteins needed for division are synthesized (microtubules)
2. Cell Division
   - Mitosis: nuclear duplicative division (4 hrs)
   - Cytokinesis: cytoplasmic division

Question 6

Mitosis: Autosomal Division

Answer 6

Parent and daughter body cells of one organism have the same exact number and type of chromosomes - they are genetically identical (duplicative division)

All division occurs continuously in some tissues such as the skin, lining of respiratory and digestive tracts and blood cells

Some cells do not usually divide at all - such as nerve and muscle cells

Question 7

Chromosomal Structure in Mitosis

Answer 7

Chromosomes contain both DNA and proteins (collectively called CHROMATIN)

During cell division only, chromosomes form 2 identical sister chromatids, which are highly condensed DNA strands (not to be confused with pairs of chromosomes)

Sister chromatids are held together at region called CENTROMERE

Question 8

The Spindle: How Chromosomes Move

Answer 8

Centrosome (an organelle inside the nucleus) organizes “spindle” of microtubules, which pull on and split the centromere and their sister chromatids

Question 9

Mitosis: Overview

Answer 9

A diploid (body or autosomal) cell makes a new, exact copy of itself

Mitosis is used in TISSUE GROWTH AND REPAIR

Question 10

Mitosis: 4 Phases

Answer 10

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

Question 11

Control of the Cell Cycle and Mitosis

Answer 11

External stimuli including hormones and growth factors stimulate the cell cycle (e.g. progesterone stimulates uterine endometrium to grow and thicken, epidermal growth factor stimulates skin to heal.)

Mediated by cell membrane receptors that relay growth signal to proteins in cytoplasm, which form a signal transduction pathway. This eventually stimulates the nucleus and activates genes that control cell division

Question 12

Meiosis: Sex Cell Division

Answer 12

Each sex cell in the ovary or testis gives rise to new cells (ova and sperm) that are NOT genetically identical to the cells they came from.

They are haploid (N)

Question 13

Meiosis: Overview

Answer 13

Two nuclear divisions make 4 haploid cells (reduction division)

Used to make gametes (eggs and sperm)

Has 8 phases (4 in each of meiosis I & II)

Question 14

Meiosis & Crossing Over: Genetic Variability

Answer 14

Synapsis of homologues

Crossing-over between sister chromatids

Chromatids after exchange have parts of the other chromatid resulting in recombinant daughter chromosomes

Question 15

Meiosis I

Answer 15

1. Prophase I: homologous chromosomes pair (synapsis) with crossing-over (exchange and mixing of genetic information assures offspring differ from parents)
2. Metaphase I: homologous pairs line up at equator. Has 2^23 or 8,388,608 different combinations of chromosomes
3. Anaphase I: homologous chromosomes separate and move towards opposite poles
4. Telophase I: 2 daughter cells result, each with 23 duplicated chromosomes

Question 16

Meiosis II

Answer 16

1. Prophase II: chromosomes condense again
2. Metaphase II: chromosomes align at the equator
3. Anaphase II: sister chromatids separate to opposite poles
4. Telophase II: 4 daughter cells result, each with 23 unduplicated chromosomes (haploid, N)

Question 17

Mitosis v Meiosis

Answer 17

Mitosis

• growth and repair
• occurs in body cells
• 1 division
• results in 2 diploid (2N) cells, both genetically identical to themselves and parent cell

Meiosis

• formation of gametes
• occurs in sex cells
• 2 divisions
• results in 4 haploids (4 ‘N’s) that are genetically different from each other and their parent cell

Question 18

Meiosis is “sexual” reproduction. Why bother?

Answer 18

Asexual reproduction (mitosis), without meiosis, occurs in unicellular organisms. One bacteria can produce 1 million offspring in about 7 hours!!

Genetic recombination results from crossing over and independent alignment of chromosomes in metaphase

Only sexual reproduction (meiosis) results in genetic recombination that ensures offspring will be genetically different than parents.

This yields genetic diversity: Survival of the fittest

Question 19

Spermatogenesis

Answer 19

• process of making sperm in males
• continual process after puberty
• about 400 million sperm are produced per day

Question 20

Oogenesis

Answer 20

• process of making eggs in females
• during meiosis 1, egg and 3 polar bodies are formed
• polar bodies act to hold discarded chromosomes and eventually disintegrate
• normally, 1 egg per month is produced ~ 400 during the entire reproductive cycle

Question 21

Abnormalities in Chromosome Number

Answer 21

Nondisjunction occurs when both members of a homologous pair go into the same daughter cell during meiosis I or when sister chromatids fail to separate in meiosis II.

Results of nondisjunction:

1. Monosomy: gamete cell has only 1 copy of a chromosome (22) e.g. Turner syndrome (only 1 X chromosome)
2. Trisomy: gamete cell has 3 copies of a chromosome (24) e.g. Down syndrome (3 copies of chromosome 21)