◦ Mitosis produces genetically identical cells for –Growth –Replacement –Asexual reproduction Copyright © 2009 Pearson Education, Inc.

MEIOSIS AND GENETIC VARIATION Copyright © 2009 Pearson Education, Inc.

◦ Somatic cells (not egg and sperm) have pairs of homologous chromosomes (one from each parent) ◦ Homologous chromosomes have the same: 1. Length 2. Centromere position 3. Gene locations Copyright © 2009 Pearson Education, Inc.

Gene locations –A locus is the position of a gene –Different versions of a gene (alleles) may be found at the same locus on maternal and paternal chromosomes Example: Chromosome 1 - contains over 3000 genes - ~90% have been determined Gene loci and alleles

◦ Sex chromosomes: X and Y  1 pair in a human  X and Y have different sizes and compositions ◦ Autosomes: are not X and Y  22 pairs in a human  Each pair has same size and composition Copyright © 2009 Pearson Education, Inc.

Sister chromatids One duplicated chromosome Centromere Homologous pair of chromosomes

–Diploid cells have two homologous sets of chromosomes (chromosome number is 2n) –Example: Somatic body cells like skin, liver, heart, blood… –Haploid cells have one set of chromosomes (chromosome number is 1n) –Example: Gametes—sperm and eggs ◦ Meiosis is a process that converts diploid nuclei to haploid nuclei Copyright © 2009 Pearson Education, Inc.

Haploid gametes (n = 23) n Egg cell Sperm cell Fertilization Meiosis Multicellular diploid adults (2n = 46) Mitosis and development n 2n2n2n2n Diploid zygote (2n = 46)

◦ Interphase ◦ Meiotic cell division –Meiosis I: homologous chromosomes separate –Produces 2 cells; the chromosome number is reduced by half –Meiosis II: sister chromatids separate –Produces 4 cells; the chromosome number remains the same Copyright © 2009 Pearson Education, Inc.

Centrosomes (with centriole pairs) PROPHASE I Microtubules attached to kinetochore INTERPHASE Sites of crossing over Metaphase plate Spindle MEIOSIS I : Homologous chromosomes separate METAPHASE I Sister chromatids remain attached ANAPHASE I Nuclear envelope Sister chromatids Centromere (with kinetochore) Homologous chromosomes separate Chromatin Tetrad

Copyright © 2009 Pearson Education, Inc. PROPHASE I MEIOSIS II : Sister chromatids separate METAPHASE II ANAPHASE II Cleavage furrow TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming TELOPHASE II AND CYTOKINESIS

◦ Outcome: –Mitosis: two genetically identical cells, with the same chromosome number as the original cell –Meiosis: four genetically different cells, with half the chromosome number of the original cell Copyright © 2009 Pearson Education, Inc.

Prophase Metaphase I Metaphase 2n = 4 Tetrads align at the metaphase plate Duplicated chromosome (two sister chromatids) Parent cell (before chromosome duplication) Chromosome duplication Chromosomes align at the metaphase plate Anaphase Telophase Sister chromatids separate during anaphase Daughter cells of mitosis 2n2n 2n2n n Chromosome duplication Site of crossing over Tetrad formed by synapsis of homologous chromosomes M EIOSIS Prophase I Anaphase I Telophase I M ITOSIS M EIOSIS I Haploid n = 2 Daughter cells of meiosis I M EIOSIS II n nn Daughter cells of meiosis II Homologous chromosomes separate (anaphase I ); sister chroma- tids remain together No further chromosomal duplication; sister chromatids separate (anaphase II )

Haploid gametes (n = 23) n Egg cell Sperm cell Fertilization Meiosis Multicellular diploid adults (2n = 46) Mitosis and development n 2n2n2n2n Diploid zygote (2n = 46)

1. Independent orientation during meiosis (chance of getting chromosome from mom or dad)  Homologous chromosomes have different “versions” or alleles of genes Copyright © 2009 Pearson Education, Inc. Brown coat (C); black eyes (E) White coat (c); pink eyes (e)

Tetrad in parent cell (homologous pair of duplicated chromosomes) Coat-color genes Chromosomes of the four gametes Meiosis Pink White Black Brown Eye-color genes C e E c C e E c C e E c

2. Random fertilization  Random egg and random sperm get together How do we get genetic variation?? Copyright © 2009 Pearson Education, Inc.

3. Crossing over  New combinations of genes are formed by genetic recombination  Material from non-sister chromatid are shared at chiasma How do we get genetic variation?? Copyright © 2009 Pearson Education, Inc.

Breakage of homologous chromatids Coat-color genes Eye-color genes C (homologous pair of chromosomes in synapsis) E ce Tetrad C E c e Joining of homologous chromatids 2 C E c e Chiasma 1

Separation of homologous chromosomes at anaphase I C E c e Chiasma Separation of chromatids at anaphase II and completion of meiosis CE c e cE C e ce c E C E C e Parental type of chromosome Gametes of four genetic types Recombinant chromosome Parental type of chromosome Recombinant chromosome 4 3

WHEN THINGS GO WRONG… ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE Copyright © 2009 Pearson Education, Inc.

◦ Karyotype: shows stained and magnified versions of chromosomes –Karyotypes are produced from dividing white blood cells, stopped at metaphase Copyright © 2009 Pearson Education, Inc. Centromere Sister chromatids Pair of homologous chromosomes 5

◦ Trisomy 21: individual inherits three copies of chromsome 21 –An imbalance in chromosome number causes Down syndrome, which is characterized by –Characteristic facial features –Susceptibility to disease –Shortened life span –Mental retardation –Variation in characteristics –The incidence increases with the age of the mother Copyright © 2009 Pearson Education, Inc.

◦ Nondisjunction : failure of chromosomes or chromatids to separate during meiosis –During Meiosis I –Both members of a homologous pair go to one pole Copyright © 2009 Pearson Education, Inc. Nondisjunction in meiosis I Normal meiosis II n + 1 Gametes Number of chromosomes n + 1n – 1

◦ Nondisjunction : failure of chromosomes or chromatids to separate during meiosis –During Meiosis II –Both sister chromatids go to one pole Copyright © 2009 Pearson Education, Inc. Nondisjunction in meiosis II Normal meiosis I Gametes Number of chromosomes n + 1n – 1n n

◦ Autosomes: usually VERY bad! ◦ Sex chromosomes: abnormalities tend to be less severe because 1. Y chromosome is small (only ~200 genes vs 2000 on X) 2. X-chromosome inactivation –In each cell of a human female, one of the two X chromosomes becomes tightly coiled and inactive Copyright © 2009 Pearson Education, Inc.

Sex linked genes/ diseases Sex determination on Y

Kleinfelter’s syndrome

Differences between Kleinfelter’s syndrome and Turner’s syndrome

◦ Structure changes result from breakage and rejoining of chromosome segments –Deletion is the loss of a chromosome segment –Cri du chat is deletion of part of chromosome 5 Copyright © 2009 Pearson Education, Inc. Deletion

Example: Cri du chat is deletion of part of chromosome 5 Copyright © 2009 Pearson Education, Inc.

Duplication Homologous chromosomes ◦ Structure changes result from breakage and rejoining of chromosome segments –Duplication is the repeat of a chromosome segment –Charcot-Marie-Tooth disease type I duplication on chromosome 17 Copyright © 2009 Pearson Education, Inc.

Example: Charcot-Marie-Tooth disease type I duplication on chromosome 17 Copyright © 2009 Pearson Education, Inc.

◦ Structure changes result from breakage and rejoining of chromosome segments –Translocation is the attachment of a segment to a nonhomologous chromosome; can be reciprocal –Some Down’s syndrome Copyright © 2009 Pearson Education, Inc. Reciprocal translocation Nonhomologous chromosomes

Example: Translocation Down’s syndrome Copyright © 2009 Pearson Education, Inc.