🧬 LAB 10 – MEIOSIS (FULL QUIZLET IMPORT VERSION)

Meiosis

Type of nuclear division that reduces chromosome number by half to form gametes.

Purpose of meiosis

To produce haploid sex cells (sperm and eggs) for sexual reproduction.

Location of meiosis

Occurs in germ cells of ovaries and testes.

Diploid (2n)

Cell with two sets of chromosomes, one from each parent.

Haploid (n)

Cell with a single set of chromosomes.

Number of cell divisions in meiosis

Two divisions (meiosis I and meiosis II).

Result of meiosis

Four non-identical haploid daughter cells.

Overall purpose

Reduction of chromosome number and creation of genetic variation.

Before meiosis begins

DNA replicates once during interphase.

Interphase in meiosis

Cell growth and DNA replication occur before meiosis I only.

Prophase I

Homologous chromosomes pair up and form tetrads (synapsis); crossing over occurs.

Synapsis

Process of homologous chromosomes pairing up side by side.

Tetrad

Structure containing four chromatids (two homologous chromosomes paired).

Crossing over

Exchange of genetic material between non-sister chromatids.

Chiasma

Visible site where crossing over has occurred.

Genetic recombination

Production of new gene combinations due to crossing over.

Metaphase I

Tetrads align at metaphase plate with random orientation.

Independent assortment

Random orientation of homologous pairs → genetic variation.

Anaphase I

Homologous chromosomes separate and move to opposite poles; sister chromatids stay attached.

Telophase I

Each pole receives one chromosome from each homologous pair → haploid set forms.

Cytokinesis I

Cytoplasm divides → two haploid cells with duplicated chromosomes.

End of meiosis I

Two haploid cells with duplicated chromosomes (2 sister chromatids per chromosome).

Interkinesis

Short pause between meiosis I and II (no DNA replication).

Prophase II

Spindle forms in each haploid cell; chromosomes condense again.

Metaphase II

Individual chromosomes align at the equator of each cell.

Anaphase II

Sister chromatids separate and move to opposite poles.

Telophase II

Nuclear membranes reform around chromosomes.

Cytokinesis II

Cytoplasm divides → four haploid daughter cells form.

Final result of meiosis

Four unique haploid cells genetically different from each other and the parent cell.

Homologous chromosomes

Chromosomes that have the same genes at the same loci but may have different alleles.

Sister chromatids

Identical copies of a single chromosome connected at a centromere.

When crossing over occurs

During Prophase I of meiosis.

When independent assortment occurs

During Metaphase I of meiosis.

Chromosome number change

Reduced from 2n → n after meiosis I; stays n after meiosis II.

Meiosis vs mitosis

Meiosis produces 4 non-identical haploid cells; mitosis produces 2 identical diploid cells.

Number of DNA replications

Once before meiosis I (never before meiosis II).

Genetic variation sources

Crossing over, independent assortment, and random fertilization.

Random fertilization

Combination of unique sperm and egg increases genetic diversity.

Purpose of crossing over

Exchange of alleles to increase genetic variation.

When homologs separate

Anaphase I.

When sister chromatids separate

Anaphase II.

Why meiosis I is called reduction division

It reduces chromosome number by half (2n → n).

Why meiosis II is called equational division

It separates sister chromatids without changing chromosome number.

Spermatogenesis

Formation of sperm cells in testes via meiosis.

Primary spermatocyte

Diploid cell that enters meiosis I.

Secondary spermatocytes

Two haploid cells formed after meiosis I.

Spermatids

Four haploid cells after meiosis II; develop into spermatozoa.

Oogenesis

Formation of egg cells in ovaries via meiosis.

Primary oocyte

Diploid cell that enters meiosis I.

Secondary oocyte

Haploid cell produced after meiosis I; receives most of cytoplasm.

Polar bodies

Small cells with little cytoplasm; usually degenerate.

End of oogenesis

One viable ovum and up to three polar bodies.

Law of segregation

Each allele pair separates during gamete formation.

Law of independent assortment

Genes for different traits segregate independently during meiosis.

Monohybrid cross

Cross involving one trait (e.g., purple × yellow kernels).

Dihybrid cross

Cross involving two traits (e.g., seed shape and color).

Punnett square

Diagram used to predict genotype and phenotype ratios.

Phenotype

Physical expression of a trait (e.g., purple kernels).

Genotype

Genetic makeup of an organism (e.g., Rr, YY).

Gametes

Sex cells carrying only one allele of each gene.

F₁ generation

First filial generation from cross of two parents.

F₂ generation

Offspring of F₁ generation; used to analyze inheritance ratios.

Independent assortment ratio in dihybrid cross

9 : 3 : 3 : 1 phenotypic ratio.

Meiosis slide identification

Look for tetrads in Prophase I and separated homologs in Anaphase I.

Crossing over microscope clue

Homologs connected at chiasmata points.

Anaphase I appearance

Paired chromosomes pulled apart, still duplicated.

Anaphase II appearance

Single chromatids pulled apart.

Telophase II appearance

Four nuclei visible with haploid sets.

Common lab organism for meiosis slides

Sordaria fimicola (ascomycete fungus).

Sordaria crossing-over experiment

Shows recombinant ascus patterns indicating gene recombination.

Purpose of meiosis lab

Visualize reduction division and genetic variation events.

Mitosis vs meiosis – number of divisions

Mitosis: 1; Meiosis: 2.

Mitosis vs meiosis – chromosome number

Mitosis: 2n → 2n; Meiosis: 2n → n.

Mitosis vs meiosis – cell type

Mitosis: Somatic; Meiosis: Gametes.

Mitosis vs meiosis – genetic outcome

Mitosis: Identical cells; Meiosis: Variable cells.

Mitosis vs meiosis – function

Mitosis: Growth and repair; Meiosis: Reproduction and variation.

Number of daughter cells

Mitosis: 2; Meiosis: 4.

Genetic similarity of products

Mitosis: Identical; Meiosis: Unique.

Crossing over

Occurs only in meiosis (Prophase I).

Independent assortment

Occurs only in meiosis (Metaphase I).