Chapter 19

Question 1

Human chromosome numbers—somatic vs. gamete?

Answer 1

Somatic cells = 46 chromosomes (23 homologous pairs); gametes = 23 single chromosomes (1 from each pair).

Question 2

Define ‘homologous chromosomes.’

Answer 2

A matched pair (one maternal, one paternal) bearing genes for the same traits at identical loci but possibly different alleles.

Question 3

Distinguish autosomes vs. sex chromosomes.

Answer 3

Autosomes = 22 homologous pairs that code for most traits; Sex chromosomes = 1 pair (XX or XY) that determine genetic sex.

Question 4

Key components of a chromosome.

Answer 4

DNA double helix + histone proteins → chromatin; condensed during mitosis; centromere binds sister chromatids; telomeres cap ends.

Question 5

Define gene & role in the cell.

Answer 5

Gene = DNA segment coding for a specific protein or RNA; product determines cell structure/function and heritable traits.

Question 6

Phases of the cell cycle in order, with major events.

Answer 6

G₁ (growth, organelles, checkpoint) → S (DNA replication) → G₂ (synthesis of mitotic machinery) → M (mitosis + cytokinesis).

Question 7

What is G₀, and which cells enter it?

Answer 7

Non‑dividing phase; neurons, mature muscle cells permanently exit cycle; liver cells can re‑enter if needed.

Question 8

Checkpoint controls—why crucial?

Answer 8

G₁/S and G₂/M checkpoints verify DNA integrity & environmental cues; failure or mutation → uncontrolled division (cancer).

Question 9

Prophase—two nuclear & two cytoplasmic changes.

Answer 9

Nuclear: chromatin condenses; nuclear envelope begins to fragment. Cytoplasmic: centrosomes separate; spindle microtubules form.

Question 10

Metaphase—what guarantees equal DNA distribution?

Answer 10

Chromosomes attach via kinetochores to opposite spindle poles and align at equatorial plate, ensuring each sister chromatid heads to a different pole.

Question 11

Anaphase—molecular trigger & outcome.

Answer 11

Enzyme separase cleaves cohesin at centromeres → sister chromatids pulled to poles via shortening spindle fibers.

Question 12

Telophase & cytokinesis—events resetting interphase state.

Answer 12

Telophase: nuclear envelopes re‑form, chromosomes decondense, spindle disassembles. Cytokinesis: actin ring forms cleavage furrow → two cells.

Question 13

Define karyotype & two clinical uses.

Answer 13

Photo array of metaphase chromosomes ordered by size/centromere; detects aneuploidy (e.g., trisomy 21) & large structural rearrangements.

Question 14

Meiosis purpose #1—ploidy maintenance across generations.

Answer 14

Meiosis halves chromosome number; fertilization restores 2 n, preventing doubling each generation.

Question 15

Meiosis purpose #2—genetic diversity mechanisms.

Answer 15

1) Crossing‑over (recombination) in prophase I; 2) Independent assortment of homologues at metaphase I; 3) Random fertilization.

Question 16

Prophase I sub‑stages (LEPTOTENE–DIPLOTENE) significance.

Answer 16

Leptotene (condense), Zygotene (synapsis via synaptonemal complex), Pachytene (crossing‑over), Diplotene (chiasmata visible), Diakinesis (prep for metaphase).

Question 17

Synapsis vs. Chiasma.

Answer 17

Synapsis: tight pairing of homologues; Chiasma: visible crossover site where nonsister chromatids exchanged segments.

Question 18

Independent assortment probability: how many combos in humans?

Answer 18

2²³ (~8.4 million) possible maternal/paternal chromosome mixes per gamete—before crossing‑over!

Question 19

Products of Meiosis I vs. Meiosis II

Answer 19

Meiosis I: 2 haploid cells, each chromosome still duplicated (sister chromatids). Meiosis II: separates chromatids → up to 4 haploid cells, each with single chromatids.

Question 20

Spermatogenesis numerical yield & location.

Answer 20

Occurs in seminiferous tubules; one primary spermatocyte → meiosis I/II → four spermatids → mature into four spermatozoa.

Question 21

Oogenesis yield & timing.

Answer 21

Primary oocyte (arrested in prophase I until puberty) → one large ovum + up to three polar bodies over two meiotic divisions; uneven cytoplasm division conserves nutrients.

Question 22

Define nondisjunction; two stages where it can happen.

Answer 22

Failure of homologues (anaphase I) or sister chromatids (anaphase II) to segregate; produces aneuploid gametes.

Question 23

Aneuploidy terms: trisomy vs. monosomy.

Answer 23

Trisomy = three copies of a chromosome (2 n + 1); monosomy = one copy (2 n – 1). Most lethal; specific viable examples: trisomy 21, XO.

Question 24

Clinical features—Down, Turner, Klinefelter.

Answer 24

Down (47, +21): intellectual disability, characteristic facial features, heart defects.
Turner (45,XO): female, short, webbed neck, sterility.
Klinefelter (47,XXY): tall male, small testes, low testosterone, possible breast enlargement.

Question 25

Why XO but not OY survives?

Answer 25

Need at least one X for vital genes; Y alone lacks many essential genes; XO viable but abnormal, OY lethal.

Question 26

Difference between crossing‑over and recombination frequency.

Answer 26

Crossing‑over: physical exchange in meiosis; recombination frequency: percentage of offspring with exchanged traits—estimates gene distance.

Question 27

Role of spindle checkpoint in meiosis I (metaphase I).

Answer 27

Ensures each homologue attached to opposite poles; prevents nondisjunction and aneuploid gametes.

Question 28

Sister chromatid vs. homologous chromosome.

Answer 28

Sisters: identical copies produced in S phase, share centromere. Homologues: same genes but different parental origin, pair only during meiosis I.

Question 29

Germ‑line mutation vs. somatic mutation impact on offspring.

Answer 29

Germ‑line mutation in gamete precursor transmitted to every cell of offspring; somatic mutation stays in individual’s body cells only.

Question 30

Cancer connection—loss of cell‑cycle control example.

Answer 30

p53 checkpoint protein mutation → cells bypass DNA‑damage arrest, accumulate mutations, proliferate uncontrollably (tumor).

Question 31

Telomeres & cell aging link.

Answer 31

Telomeres shorten each division; when critically short, cell enters senescence; telomerase in germ/stem/cancer cells maintains length allowing limitless divisions.

Question 32

Why crossing‑over rarely occurs in mitosis.

Answer 32

Homologous chromosomes do not synapse in mitosis; sisters remain paired, homologues act independently → no opportunity for inter‑homologue exchange.