Meiosis 2011 12 Meiosis Halves the Chromosome Number

Meiosis 2011 -12

Meiosis: Halves the Chromosome Number • Special type of cell division • Used only for sexual reproduction • Halves the chromosome number prior to fertilization – Parents diploid – Meiosis produces haploid gametes – Gametes fuse in fertilization to form diploid zygote – Becomes the next diploid generation 2011 -12

One process, three aspects • Meiosis • Sexual reproduction • Heredity 2011 -12

Heredity • DNA • Genes = specific sequences of nucleotides • Inheritance possible because – a. DNA replication is an exact process – b. Sperm and ova are combined in nucleus of fertilized egg • Transmission of genes depends on behavior of chromosomes 2011 -12

Chromosome • Single long DNA molecule; highly folded and coiled with proteins • Contains genetic information arranged in linear fashion • Contains 100 s or 1000 s of genes, each in a specific region of DNA; locus • Each species has a specific number of chromosomes 2011 -12

Homologous Pairs of Chromosomes • In diploid body cells chromosomes occur in pairs • Humans have 23 different types of chromosomes • Diploid cells have two of each type • Chromosomes of the same type are said to be homologous – They have the same length – Their centromeres are positioned in the same place – One came from the father (the paternal homolog) the other from the mother (the maternal homolog) – When stained, they show similar banding patterns – Because they have genes controlling the same traits at the same positions 2011 -12

Homologous Chromosomes 2011 -12

Homologous Pairs of Chromosomes • Homologous chromosomes have genes controlling the same trait at the same position – Each gene occurs in duplicate – A maternal copy from the mother – A paternal copy from the father • Many genes exist in several variant forms in a large population • Homologous copies of a gene may encode identical or differing genetic information • The variants that exist for a gene are called alleles • An individual may have: – Identical alleles for a specific gene on both homologs (homozygous for the trait), or – A maternal allele that differs from the corresponding paternal allele (heterozygous for the 2011 -12 trait)

Asexual vs. sexual reproduction Asexual Reproduction Single individual is sole parent. Two parents give rise to offspring. Single parent passes on all its genes to its offspring. Each parent passes on half its genes to its offspring. Offspring are genetically identical to the parent. Offspring have a unique combination of genes inherited from both parents Results in a clone. Rarely differences occur and as a result of mutation. Results in greater genetic variation; offspring vary genetically from their siblings and parents. 2011 -12

Sexual Life Cycles Alternation of generations The only haploid cells spores The only diploid stage 2011 -12

Terms to review • Life cycle • Somatic cell • Homologous chromosome pairs (homologues) – Except for X/Y they carry same genetic loci (locus) • Karyotype – Use human lymphocytes – Can find chromosomal abnormalities • Diploid (2 n) • Haploid (n); gametes – Sperm cells/ova (23 chromosomes) – Only cells in body not produced by mitosis • Fertilization • Zygote • Autosomes (22 pairs) • Sex chromosomes (1 pair)2011 -12

Overview of Meiosis 2011 -12

Phases of Meiosis I: Prophase I & Metaphase I • Meiosis I (reductional division): –Prophase I • Each chromosome internally duplicated (consists of two identical sister chromatids) • Homologous chromosomes pair up – synapsis • Physically align themselves against each other end to end • End view would show four chromatids – Tetrad –Metaphase I • Homologous pairs arranged onto the metaphase plate 2011 -12

Phases of Meiosis I: Anaphase I & Telophase I • Meiosis I (cont. ): –Anaphase I • Synapsis breaks up • Homologous chromosomes separate from one another • Homologues move towards opposite poles • Each is still an internally duplicate chromosome with two chromatids –Telophase I • Daughter cells have one internally duplicate chromosome from each homologous pair • One (internally duplicate) chromosome of each type (1 n, haploid) 2011 -12

Phases of Meiosis I: Cytokinesis I & Interkinesis • Meiosis I (cont. ): –Cytokinesis I • Two daughter cells • Both with one internally duplicate chromosome of each type • Haploid • Meiosis I is reductional (halves chromosome number) • Interkinesis –Similar to mitotic interphase –Usually shorter –No replication of DNA 2011 -12

Genetic variations • Sexual life cycles --> variations – Independent assortment of chromosomes – Crossing over during prophase I – Random fusion of gametes during fertilization 2011 -12

Genetic Variation: Crossing Over • Meiosis brings about genetic variation in two key ways: – Crossing-over between homologous chromosomes, and – Independent assortment of homologous chromosomes • 1. Crossing Over: – Exchange of genetic material between nonsister chromatids during meiosis I – At synapsis, a nucleoprotein lattice (called the synaptonemal complex) appears between homologues • Holds homologues together • Aligns DNA of nonsister chromatids • Allows crossing-over to occur – Then homologues separate and are distributed to different daughter 2011 -12 cells

Crossing Over 2011 -12

Crossing over Protein strands hold homologues in register with each other Protein cross bridges DNA loops 2011 -12

Crossing over (2) Harlequin chromosomes demonstrate reciprocal exchange between sister chromatids 2011 -12

Genetic Variation: Independent Assortment • 2. Independent assortment: –When homologues align at the metaphase plate: • They separate in a random manner • The maternal or paternal homologue may be oriented toward either pole of mother cell –Causes random mixing of blocks of alleles into gametes 2011 -12

Independent Assortment 2011 -12

Independent assortment Homologous pairs behave independently of each other Consider the possible combinations of 23 pairs ! Fertilization = (223)2 = 64 x 1012 variations 2011 -12 2 n = 223 = 8 x 106 (1 parent)

Recombination 2011 -12

Genetic Variation: Fertilization • When gametes fuse at fertilization: –Chromosomes donated by the parents are combined –In humans, (223)2 = 70, 368, 744, 000 chromosomally different zygotes are possible • If crossing-over occurs only once – (423)2, or 4, 951, 760, 200, 000, 000 genetically different zygotes are possible 2011 -12

Genetic Variation: Significance • Asexual reproduction produces genetically identical clones • Sexual reproduction cause novel genetic recombinations • Asexual reproduction is advantageous when environment is stable • However, if environment changes, genetic variability introduced by sexual reproduction may be advantageous 2011 -12

Phases of Meiosis II: Similar to Mitosis • Metaphase II –Overview • Unremarkable • Virtually indistinguishable from mitosis of two haploid cells –Prophase II – Chromosomes condense –Metaphase II – chromosomes align at metaphase plate 2011 -12

– Anaphase II • Centromere dissolves • Sister chromatids separate and become daughter chromosomes – Telophase II and cytokinesis II • Four haploid cells • All genetically unique 2011 -12

Meiosis I & II in Plant Cells 2011 -12

Meiosis versus Mitosis • Mitosis • Meiosis – Requires one nuclear division – Requires two nuclear divisions – Chromosomes do not synapse nor cross over – Chromosomes synapse and cross over – Centromeres survive Anaphase I – Centromeres dissolve in mitotic anaphase – Halves chromosome number – Preserves chromosome number – Produces four daughter nuclei – Produces two daughter nuclei – Produces daughter cells genetically different from parent and each other – Produces daughter cells genetically identical to parent and to each other – Used only for sexual reproduction – Used for asexual reproduction and growth 2011 -12

Meiosis Compared to Mitosis 2011 -12

2011 -12

Meiosis I Compared to Mitosis 2011 -12

Meiosis II Compared to Mitosis 2011 -12

Life Cycle Basics: Plants • Haploid multicellular “individuals” alternate with diploid multicellular “individuals” • The haploid individual: –Known as the gametophyte –May be larger or smaller than the diploid individual • The diploid individual: –Known as the sporophyte –May be larger or smaller than the haploid individual 2011 -12

Life Cycle Basics: Plants • Mosses are haploid most of their life cycle • Ferns & higher plants have mostly diploid life cycles • In fungi and most algae, only the zygote is diploid • In plants, algae, & fungi, gametes produced by haploid individuals 2011 -12

Life Cycle Basics: Animals • In familiar animals: –“Individuals” are diploid; produce haploid gametes –Only haploid part of life cycle is the gametes –The products of meiosis are always gametes 2011 -12

Life Cycle Basics: Animals – Meiosis occurs only during gametogenesis • Production of sperm –Spermatogenesis –All four cells become sperm • Production of eggs –Oogenesis –Only one of four nuclei get cytoplasm » Becomes the egg or ovum » Others wither away as polar bodies 2011 -12

The Human Life Cycle • Sperm and egg are produced by meiosis • A sperm and egg fuse at fertilization • Results in a zygote –The one-celled stage of an individual of the next generation –Undergoes mitosis • Results in multicellular embryo that gradually takes on features determined when zygote was formed 2011 -12

The Human Life Cycle • All growth occurs as mitotic division • As a result of mitosis, each somatic cell in body – Has same number of chromosomes as zygote – Has genetic makeup determined when zygote was formed 2011 -12

The Human Life Cycle 2011 -12

Gametogenesis in Mammals 2011 -12

Oogenesis Long interkinesis 2011 -12

Spermatogenesis 2011 -12

Meiosis 2011 -12

Mitosis 2011 -12