Meiosis and Sexual Life Cycles

Describe the different Inheritance aspects

Heredity = passing traits from parents → offspring through genetic material.

Variation = differences between members of the same species.

Genetics = study of heredity and hereditary variation.

Genes are DNA regions containing instructions for a specific RNA or protein. Genes passed down cause similarities, while different versions of genes cause variation. A gene’s specific chromosome location is its locus; loci = plural.

Types of reproduction and how they both work

Asexual reproduction: 1 parent → mitosis → genetically identical offspring called clones. Used by bacteria, single-celled eukaryotes, and some multicellular eukaryotes.

Sexual reproduction: 2 parents contribute genes. A diploid cell → meiosis → haploid gametes → fertilization combines 2 gametes → new diploid cell. Offspring are genetically unique.

How can some species reproduce both sexually and asexually?

Some species can switch between both methods.

Coast redwoods: sexually through seeds; asexually when new trees sprout from the root crown, stump, or fallen branches.

Komodo dragons and zebra sharks: sexually or through parthenogenesis, where an unfertilized egg develops into an embryo without fertilization.

Describe Chromosome Number + Diploid vs. Haploid

Each species has a characteristic chromosome number, which is the number of chromosomes/DNA molecules making up its genome.

It does not indicate size or complexity.

Diploid (2n) = 2 copies of each chromosome; most somatic cells are diploid.

Haploid (n) = 1 copy of each chromosome; gametes are generally haploid.

Easy: diploid = 2 copies, haploid = 1 copy.

What are Homologous Chromosomes?

Homologous chromosomes (homologues) are chromosome pairs in diploid cells.

They are similar in size and structure and contain the same genes at the same loci, but they can have different alleles.

Alleles = different versions of the same gene.

Sister vs. Non-Sister Chromatids

Sister chromatids = 2 joined copies of the same duplicated chromosome. Non-sister chromatids = chromatids belonging to different homologous chromosomes.

Easy: same chromosome → sisters; different homologues → non-sisters.

What is a Karyotype?

A karyotype is an organized display of an organism's homologous chromosome pairs, usually arranged by size.

Chromosomes are viewed during metaphase of mitosis because they are highly condensed and easy to see.

What are autosomes?

Autosomes = chromosomes that are not sex chromosomes.

Humans have 22 homologous pairs of autosomes. Humans have 23 chromosome pairs total: 22 autosome pairs + 1 sex chromosome pair.

What are sex chromosomes and how do they work?

The X and Y chromosomes are involved in determining biological sex.

XX: the chromosomes are homologous.

XY: X and Y are not fully homologous because they differ greatly in size and genes.

The X is much larger than Y, but small homologous regions allow X and Y to pair during meiosis.

Animal Sexual Life Cycle

Most animals, including humans: gametes (n) → fertilization → diploid zygote (2n) → mitosis → multicellular diploid adult → germ-line cells undergo meiosis → haploid gametes (n).

Gametes are the only haploid cells; all other cells are diploid. The mitochondria in the zygote initially come from the egg, not the sperm.

Sexual Life Cycles – Plants and Some Algae

Called alternation of generations because there is both a multicellular diploid and multicellular haploid stage.

Diploid organism → meiosis → haploid spores → mitosis → haploid organism → mitosis → gametes → fertilization → diploid zygote → mitosis → diploid organism.

Important: meiosis produces spores, not gametes. Gametes are made by mitosis

Fungi & Some Protists Sexual Life Cycle

The zygote is the only diploid stage; there is no multicellular diploid organism.

Gametes (n) → fertilization → zygote (2n) → meiosis immediately → haploid cells → mitosis → haploid organism/cells → mitosis → gametes.

The diploid zygote does not grow by mitosis.

Meiosis I – Prophase I

1. Chromosomes condense.

2. Spindle forms.

3. Nuclear envelope breaks down.

4. Homologues pair with matching loci aligned, forming a tetrad: 2 homologous chromosomes = 4 chromatids.

5. Cohesins hold sister chromatids together.

6. Non-sister chromatids break at matching locations.

7. Synaptonemal complex holds homologues together; this pairing = synapsis.

8. Non-sister chromatids reconnect and exchange DNA = crossing over.

9. This forms recombinant chromatids with new allele combinations.

10. After the complex breaks down, homologues remain connected at chiasmata. Spindle microtubules attach to kinetochores.

Meiosis I – Metaphase I

Homologous chromosome pairs line up at the metaphase plate.

Each homologue connects to spindle microtubules from opposite poles. Sister chromatids stay together; their kinetochores act as one functional unit so both sisters attach toward the same pole.

Meiosis I – Anaphase I

Cohesins along the chromosome arms break, allowing homologous chromosomes to separate and move to opposite poles. Cohesins at the centromeres stay intact, so the sister chromatids remain attached and move together.

Key: homologues separate; sisters do NOT.

Telophase I of Meiosis I

New nuclear envelopes form around chromosomes at each pole and cytokinesis divides the cell into 2 cells.

Each cell is now haploid (n) because it has only one chromosome from each homologous pair. However, every chromosome is still duplicated, containing 2 sister chromatids. meiosis

Meiosis II

Meiosis II is similar to mitosis.

1. Nuclear envelope breaks down.

2. Chromosomes line up at the metaphase plate.

3. Centromere cohesins break.

4. Sister chromatids separate to opposite poles; once separated, each becomes an individual chromosome.

5. Nuclear envelopes reform.

6. Cytokinesis occurs.

Final: 1 diploid cell → 2 haploid cells after meiosis I → 4 genetically unique haploid cells after meiosis II.

Genetic Variation in Sexual Reproduction

A major advantage of sexual reproduction is genetic variation from new combinations of alleles. This gives natural selection more differences to act on.

If the environment changes, some individuals may have allele combinations that help them survive and reproduce.

Three factors contribute to genetic variability and how they work

1. Independent assortment: homologous pairs line up randomly and independently during metaphase I, producing different chromosome combinations in gametes. Number of combinations = 2ⁿ; humans: 2²³ ≈ 8.4 million.

2. Random fertilization: any sperm can potentially fertilize any egg; 8.4 million × 8.4 million ≈ 70 trillion possible chromosome combinations.

3. Crossing over: during prophase I, non-sister chromatids exchange DNA, creating recombinant chromosomes with new combinations of alleles. Humans usually have about 1–3 crossover events/chromosome.