Meiosis AP Biology

Hereditary Similarity and Variation Heredity is the transmission of traits from one generation to the next Variation shows that offspring differ in appearance from parents and siblings Genetics is the scientific study of heredity and variation

Genes Genes are the units of heredity Genes are segments of DNA Each gene has a specific locus on a certain chromosome One set of chromosomes is inherited from each parent

Sexual/Asexual Sexual reproduction- two parents give rise to offspring that have unique combinations of genes inherited from the two parents Asexual reproduction- one parent produces genetically identical offspring by mitosis

Karyotype karyotype -ordered display of the pairs of chromosomes from a cell homologous chromosomes (homologues) Both chromosomes in a pair. They carry genes controlling the same inherited characteristics – Alleles-

LE µm Pair of homologous chromosomes Sister chromatids Centromere

LE 13-4 Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosomes Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) Centromere “Hey, that’s my chromosome!”

Homologous Chromosomes Each pair of homologous chromosomes includes one chromosome from each parent For humans, the diploid number is 46 (2n = 46) 23 Pairs. Gametes are haploid cells, containing only one set of chromosomes

Meiosis: twice as nice Meiosis I Synapsis Crossing over Tetrad formation Meiosis II same as mitosis sister chromatids split 4 unique cells result

LE 13-7 Homologous pair of chromosomes in diploid parent cell Interphase Homologous pair of replicated chromosomes Chromosomes replicate Meiosis I Diploid cell with replicated chromosomes Sister chromatids Meiosis II Homologous chromosomes separate Sister chromatids separate Haploid cells with replicated chromosomes Haploid cells with unreplicated chromosomes

LE 13-8ab Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Microtubule attached to kinetochore Tetrad MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example Pairs of homologous chromosomes split up Tetrads line up

Beginning of telophase I- each half of the cell has a haploid set of chromosomes – each chromosome still consists of two sister chromatids Cytokinesis usually occurs simultaneously, forming two haploid daughter cells

LE 13-8b Cleavage furrow MEIOSIS II: Separates sister chromatids PROPHASE II METAPHASE IIANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming Two haploid cells form; chromosomes are still double During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes

Crossing Over Because of crossing over in Prophase I, the two sister chromatids of each chromosome are no longer genetically identical Anaphase I: Homologous chromosomes separate. sister chromatids stay together Anaphase II: sister chromatids separate The sister chromatids of each chromosome are now two newly individual chromosomes.

Four for the Price of One At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes Each daughter cell is genetically distinct from the others and from the parent cell

LE 13-9 Propase Duplicated chromosome (two sister chromatids) Chromosome replication 2n = 6 Parent cell (before chromosome replication) Chromosome replication MITOSISMEIOSIS Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Tetrads positioned at the metaphase plate Metaphase I Chromosomes positioned at the metaphase plate Metaphase Anaphase Telophase Homologues separate during anaphase I; sister chromatids remain together Sister chromatids separate during anaphase Daughter cells of meiosis I Haploid n = 3 Anaphase I Telophase I MEIOSIS II Daughter cells of mitosis 2n2n 2n2n n Sister chromatids separate during anaphase II n nn Daughter cells of meiosis II

Genetic variation contributes to Evolution Mutations create different versions of genes Reshuffling of different versions of genes during sexual reproduction produces genetic variation Three mechanisms for variation in Sexual: – Independent assortment of chromosomes – Crossing over (prophase I) – Random fertilization

Independent Assortment Independent assortment-each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs (not all dad’s chroms to 1 daughter! And mom’s to the other) For humans (n = 23), there are more than 8 million (2 23 ) possible combinations of chromosomes

Crossing over to the other side Crossing over produces recombinant chromosomes, which combine genes inherited from each parent In crossing over, homologous portions of two nonsister chromatids trade places Prophase I

LE Prophase I of meiosis Tetrad Nonsister chromatids Chiasma, site of crossing over Recombinant chromosomes Metaphase I Metaphase II Daughter cells

Random fertilization adds to genetic variation because any sperm can fuse with any ovum The fusion of gametes produces a zygote with any of about 64 trillion diploid combinations Crossing over adds even more variation