MEIOSIS & GAMETOGENESIS

Bio Exam 4

What is meiosis?

A type of cell division that reduces chromosome number from diploid (2n) to haploid (n), producing gametes.

Ploidy

The number of chromosome sets a cell has.

• Haploid (n): 1 set (gametes)

• Diploid (2n): 2 sets (somatic cells)

Why do different organisms spend different amounts of time in haploid/diploid states?

Each species has a unique life cycle strategy (e.g., humans mostly diploid, fungi mostly haploid).

How many chromosomes do humans have?

• 23 pairs of homologous chromosomes

• 22 autosome pairs

• 1 sex chromosome pair (XX or XY)

Homologous Chromosomes

• Same genes in same locations

• Same length

• Same centromere position
  One homolog comes from the egg, the other from the sperm.

What happens in interphase before meiosis?

Same as mitosis interphase:

• G1: growth

• S: DNA replication

• G2: preparation
  Each chromosome becomes two sister chromatids.

What is the main outcome of meiosis I?

Separation of homologous chromosomes, reducing ploidy from 2n → n

Prophase I

• Chromosomes condense

• Nuclear membrane breaks down

• Spindle fibers form

• Homologous chromosomes pair into bivalents/tetrads

• Crossing over occurs

• Synaptonemal complex forms, then breaks

• Chiasmata remain where crossover happened

What is crossing over?

Exchange of DNA segments between non-sister chromatids, producing recombinant chromatids → genetic variation.

Why is crossing over important?

It creates new allele combinations not present in either parent

Bivalent / Tetrad

A structure of 2 homologous chromosomes paired together (4 chromatids total).

What happens in metaphase I?

Tetrads align at the metaphase plate.
Random orientation of maternal vs paternal homologs creates variation.

Anaphase I

Cohesin holding homologs breaks down (but centromere cohesin remains).
Homologs separate → move to opposite poles.

What happens in telophase I & cytokinesis?

• Chromosomes reach poles

• Cell divides

• Two haploid daughter cells are formed
  Chromosomes still consist of two sister chromatids.

Is DNA replicated again before meiosis II?

No — DNA is not replicated again.

Prophase II

• Spindle reforms

• Chromosomes may re-condense if needed

• Kinetochore microtubules attach to each sister chromatid

What happens in metaphase II?

Sister chromatids line up at the metaphase plate.
They may be non-identical due to crossing over.

Anaphase II

Cohesin finally degrades → sister chromatids separate

What happens in telophase II and cytokinesis?

• Nuclear membranes reform

• Chromosomes decondense

• Cells divide
  Result: Four genetically distinct haploid cells

What are the two major sources of variation in meiosis?

Crossing over (Prophase I)

Independent assortment of homologs (Metaphase I)

Why are haploid cells from meiosis genetically unique?

Because each one gets a random mix of maternal/paternal chromosomes, many of which are now recombinant.

Spermatogenesis

Meiosis in males; one diploid spermatocyte → four equal, motile sperm.
Spermatids lose cytoplasm and develop a flagellum.

Oogenesis

Meiosis in females; one diploid oocyte → one large egg + three polar bodies (due to unequal cytoplasmic division).

Why does the egg receive most of the cytoplasm?

It needs stored nutrients, organelles, and molecules to support early embryonic development

When does female meiosis begin and pause?

• Begins in fetal development

• Arrests in Prophase I until puberty

• After puberty: completes Meiosis I monthly and arrests in Metaphase II

• Meiosis II only completes upon fertilization

What is nondisjunction?

Failure of homologs or sister chromatids to separate → gametes with abnormal chromosome numbers (aneuploidy)

What happens if nondisjunction occurs in meiosis I?

• Homologs fail to separate

• 100% of gametes abnormal:

  • Half: n+1

  • Half: n–1

What happens if nondisjunction occurs in meiosis II?

• Sister chromatids fail to separate

• 50% abnormal gametes (n+1 or n–1)

• 50% normal gametes (n)

Trisomy

2n + 1 chromosomes
Example: Trisomy 21 (Down Syndrome).

Monosomy

2n – 1 chromosomes
Example: Turner Syndrome (XO) — the only viable human monosomy.

Polyploidy

More than two complete sets of chromosomes (3n, 4n, etc.).
Often lethal in animals but tolerated in plants

How can polyploidy occur?

• Gamete produced as 2n due to massive nondisjunction

• Fertilized by normal n gamete → 3n zygote

• Or a diploid cell duplicates DNA but fails cytokinesis → 4n cel

What are the 3 major events that create genetic uniqueness in offspring?

• Crossing over

• Independent assortment

• Random fertilization (not on exam sheet, but implied by genetics)