Meiosis and Sexual Life Cycles Chapter 13 Meiosis and Sexual Life Cycles

Chromosomes

Chromosomes

Chromosomes

Chromosomes

Karyotype

Chromosomes Nucleosome Chromosome DNA double helix Coils Supercoils 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.

Sexual Life Cycles of Plants Gametophyte (N) Sporophyte (2N) Bryophytes Ferns Seed 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 Over Video

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…