Chapter 13

Meiosis and Sexual Life Cycles

Chromosomes

Chromosomes

Chromosomes

Chromosomes

Karyotype

ChromosomesChromosome

Supercoils

Coils

Nucleosome

Histones

DNA

double

helix

Chromosomes• Genes are units of hereditary information that

are passed down from parent to offspring.• The tens of thousands of genes (alleles) you

inherit from you mother and father make up your genome.

• A gene’s specific location on a chromosome is called the gene’s locus.

Asexual Life Cycles• Asexual reproduction – A single parent

passes copies of all its genes to its offspring.• Examples in animals: hydra (budding),

starfish (regeneration), Daphnia (parthenogenesis)

Asexual Life Cycles• Asexual reproduction • Examples in plants: underground stem

(tubers and bulbs), leaves (form tiny plantlets that fall off and root), roots (aspen send up shoots from their roots – largest organism on Earth)

Sexual Life Cycles

• Sexual reproduction – two parents give rise to offspring that have unique combinations of genes (alleles) inherited from the two parents

• Results in greater variation because there is a combination of genes (alleles).

• Can increase adaptation and success because some genes (alleles) can be “better” than others.

Can You Match the Offspring With Their Siblings and Parents?

The Human Life Cycle• Somatic cells – Any cell that is not sperm or

ovum. All humans have 46 chromosomes in each somatic cell.

• Homologous chromosomes (homologues) – Chromosomes that make up a pair; have the same length, centromere position, and staining pattern.

• Sex chromosomes – X and Y chromosomes• Autosomes – all other chromosomes• Our 46 chromosomes are actually two sets of

23 chromosomes – one set from mom, one set from dad

The Human Life Cycle• Gametes – sex cells or reproductive cells;

have 22 autosomes and one sex chromosomes

• Haploid cell – a cell with a single chromosome set (sperm and egg). For humans, the haploid number is 23 (n = 23)

• Fertilization – the fusion of egg and sperm• Zygote – the fertilized egg; contains two sets

of chromosomes.• Diploid cell – a cell with two sets of

chromosomes. For humans, the diploid number is 46 (2n = 46)

The Human Life Cycle• Gametes (sperm and egg) are the only

cells not produced by mitosis.• Instead, gametes are formed by

meiosis.• In meiosis, the number of chromosomes

is halved so each new gamete receives half the original number of chromosomes.

• In humans, 2n = 46, n = 23

The Human Life Cycle

A Variety of Sexual Life Cycle

• In most animals, meiosis only occurs during the production of gametes.

• After fertilization, the diploid zygote divides by mitosis producing a multicellular, diploid organism.

A Variety of Sexual Life Cycle

• In most fungi and some protists, gametes fuse to form a diploid zygote and then meiosis occurs to form a multicellular, adult haploid organism.

• The haploid organism then produces gametes by mitosis to form a diploid zygote.

A Variety of Sexual Life Cycle• Plants and some algae

exhibit alternation of generation.

• The multicellular, diploid stage is the sporophyte – produces haploid cells called spores.

• The spore divides mitotically to produce the multicellular stage called gametophyte – makes gametes by mitosis which fuse to produce the zygote.

Gametophyte

(N)

Sporophyte (2N)

Bryophytes Ferns Seed plants

Sexual Life Cycles of Plants

Meiosis• Meiosis reduces chromosome number from

diploid to haploid.• Occurs in two divisions called meiosis I and

meiosis II.• Divisions result in 4 daughter cells, each with

half the number of chromosomes as the parent.

• Homologous chromosomes – individual chromosomes that were inherited from different parents.

Chromosomes

Meiosis

Meiosis I

Meiosis II

Meiosis – Interphase

• Chromosomes replicate • Sister chromatids remain

attached at the centromere

• Centrosomes replicate

Meiosis – Prophase I• Chromosomes begin to

condense• Homologous chromosomes pair

up forming a tetrad• Chiasmata hold homologous

pairs together – crossing over usually occurs

• Centrosomes move to opposite poles

• Spindle microtubules begin to form and attach to the centromere of the chromosomes

• Nuclear membrane and nucleoli disappear

Meiosis – Metaphase I• Chromosomes are arranged

on the metaphase plate, still in homologous pairs

• Kinetochore microtubules from one pole of the cell are attached to one chromosome of each pair, while microtubules from the opposite pole are attached to the homologue.

Meiosis – Anaphase I

• Sister chromatids remain attached at their centromere and move as a single unit toward the same pole

• The homologous chromosome moves towards the opposite pole.

Meiosis – Telophase I & Cytokinesis• Chromosomes continue to move to

opposite poles• Each pole now has a haploid set of

chromosomes• Each chromosome still has sister

chromatids attached at the centromere

• During cytokinesis, a cleavage furrow occurs in animal cells and a cell plate occurs in plant cells.

• No new genetic material is made before the beginning of meiosis II.

Meiosis – Prophase II

• A spindle apparatus forms

• The chromosomes move towards the metaphase II plate.

Meiosis – Metaphase II

• Chromosomes are positioned on the metaphase plate with the kinetochore of sister chromatids of each chromosome pointing toward opposite poles.

Meiosis – Anaphase II

• The centromere of sister chromatids finally separate

• Sister chromatids of each pair, now individual chromosomes, move toward opposite poles of the cell.

Meiosis – Telophase II and Cytokinesis

• Nuclei form at opposite poles of the cell and cytokinesis occurs.

• At the completion of cytokinesis, there are four haploid daughter cells

• Each new cell is NOT identical.

Videos

Comparing Mitosis and Meiosis

Comparing Mitosis and Meiosis

Origins of Genetic Variation

Independent Assortment of Chromosomes

• Each gamete that a human produces contains one of 8 million (223)possible assortments of chromosomes inherited from that individual’s parents.

Independent Assortment of Chromosomes

Origins of Genetic Variation

Crossing Over

• Crossing over produces recombinant chromosomes which combine genes inherited from both parents.

• Occurs early in prophase I• Homologous portions of two non-sister

chromatids trade places

Crossing Over

Crossing OverVideo

Origins of Genetic Variation

Random Fertilization

• A human egg cell (ovum) represents about 8 million possible chromosome combinations.

• A human sperm cell represents about 8 million possible chromosome combinations.

• Any two parents will produce a zygote with about 64 trillion different combinations (that’s not even counting the variation from crossing over!)

Origins of Genetic Variation• Sexual life cycles produce genetic

variation among offspring• Evolutionary adaptation depends on a

population’s genetic variation• Consider the story of the Daphnia…