Chapter 13 Overview: Hereditary Similarity and Variation Living organisms Are distinguished by their ability to reproduce their own kind WARNING: This chapter relies heavily on a knowledge of definitions. Terminology is crucial to understanding meiosis.

Heredity Is the transmission of traits from one generation to the next Variation Shows that offspring differ somewhat in appearance from parents and siblings

Genetics Is the scientific study of heredity and hereditary variation

Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes

Inheritance of Genes Genes Are the units of heredity Are segments of DNA Segments of nucleotides (A, T, C, G) arranged in sequence

Each gene in an organism’s DNA Has a specific locus on a certain chromosome Each chromosome is DNA and proteins May have 100’s or 1000’s of genes We inherit One set of chromosomes from our mother and one set from our father

Comparison of Asexual and Sexual Reproduction In asexual reproduction One parent produces genetically identical offspring by mitosis Figure 13.2 Parent Bud 0.5 mm This gives rise to clones – a group of genetically identical individuals

In sexual reproduction Two parents give rise to offspring that have unique combinations of genes inherited from the two parents

Fertilization and Meiosis Concept 13.2: A life cycle Is the generation-to-generation sequence of stages in the reproductive history of an organism

Sets of Chromosomes in Human Cells In humans Each somatic cell has 46 chromosomes, made up of two sets One set of chromosomes comes from each parent Somatic cells – all the cells other than sperm or egg

A karyotype Is an ordered, visual representation of the chromosomes in a cell 5 µm Pair of homologous chromosomes Centromere Sister chromatids Figure 13.3

Homologous chromosomes Are the two chromosomes composing a pair Have the same characteristics (length, staining pattern, position of centromere) May also be called autosomes

Sex chromosomes Are distinct from each other in their characteristics Are represented as X and Y Determine the sex of the individual, XX being female, XY being male

A diploid cell Has two sets of each of its chromosomes One set came from mom, one from dad Humans have 46 chromosomes (2n = 46) n = the number of chromosomes in a set

In a cell in which DNA synthesis has occurred Remember . . . In a cell in which DNA synthesis has occurred All the chromosomes are duplicated and thus each consists of two identical sister chromatids Figure 13.4 Key Maternal set of chromosomes (n = 3) Cell is in G2 phase 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)

Unlike Somatic Cells, Gametes - sperm and egg cells, are haploid cells, containing only one set of chromosomes The haploid number is n = 23 in humans 22 autosomes 1 single sex chromosome

Behavior of Chromosome Sets in the Human Life Cycle At sexual maturity The ovaries and testes produce haploid gametes by meiosis

During fertilization The zygote These gametes fuse, forming a diploid zygote The zygote Develops into an adult organism

The human life cycle Key Figure 13.5 Haploid gametes (n = 23) Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)

Meiosis – Reduces the Number of Chromosomes Concept 13.3: Meiosis Takes place in two sets of divisions, meiosis I and meiosis II Results in 4 daughter cells, each genetically unique

The Stages of Meiosis An overview of meiosis Figure 13.7 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes 1 2 Homologous separate Haploid cells with replicated chromosomes Sister chromatids Haploid cells with unreplicated chromosomes Meiosis I Meiosis II

Meiosis I Reduces the number of chromosomes from diploid to haploid Meiosis II Produces four haploid daughter cells

Details of Meiosis Stages CAUTION! Stages of meiosis I and II use the same names as stages in mitosis There are differences in these stages though!

Stages of Meiosis Chromosomes replicated in S phase

Stages of Meiosis I Chromosomes begin to condense Homologous chromosomes loosely pair along their lengths In crossing over, DNA of nonsister chromatids break and rejoin to the other’s DNA In synapsis homologues are held tightly along length Tetrads – group of 4 chromosomes are visible Also, all the happenings of Prophase seen in mitosis occur too

Stages of Meiosis I Pairs of homologous chromosomes (in tetrads) move to the middle of the metaphase plate Both chromatids of a homologous chromosome are attached to one kinetochore microtubule

Stages of Meiosis I Chromosomes move to opposite poles Sister chromatids remain attached at centromere

Stages of Meiosis I Each cell has a haploid set of chromosomes, but each chromosome still has sister chromatids No chromosome replication takes place before meiosis II

Stages of Meiosis II Spindles form By end of Prophase II, they attach at centromere

Stages of Meiosis II Chromosomes moved to the middle of metaphase plate Sister chromatids are not genetically identical

Stages of Meiosis II Sister chromatids come apart, move to opposite poles

Stages of Meiosis II Meiotic division of 1 cell produces 4 genetically different daughter cells Each daughter cell has a haploid number of chromosomes

A Comparison of Mitosis and Meiosis Meiosis I and II can be distinguished from mitosis because: Meiosis reduces the number of chromosome sets Mitosis conserves the number of chromosome sets And also by: By three events in Meiosis l

1) Synapsis and crossing over Homologous chromosomes physically connect and exchange genetic information

2) Tetrads on the metaphase plate At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates

3) Separation of homologues At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell In anaphase II of meiosis, the sister chromatids separate

(before chromosome replication) A comparison of mitosis and meiosis Figure 13.9 MITOSIS MEIOSIS Prophase Duplicated chromosome (two sister chromatids) Chromosome replication Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Metaphase Chromosomes positioned at the metaphase plate Tetrads Metaphase I Anaphase I Telophase I Haploid n = 3 MEIOSIS II Daughter cells of meiosis I Homologues separate during anaphase I; sister chromatids remain together Daughter cells of meiosis II n Sister chromatids separate during anaphase II Anaphase Telophase Sister chromatids separate during anaphase 2n Daughter cells of mitosis 2n = 6 Text box of Figure 13.9 is excellent!

Genetic Variation and Evolution Concept 13.4: Reshuffling of genetic material in meiosis Produces genetic variation

Origins of Genetic Variation Among Offspring In species that reproduce sexually The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation

1) Independent Assortment of Chromosomes Homologous pairs of chromosomes Orient randomly at metaphase I of meiosis

In independent assortment Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs Key Maternal set of chromosomes Paternal set of Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Possibility 2 Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 Figure 13.10

2) Crossing Over Crossing over Produces recombinant chromosomes that carry genes derived from two different parents Figure 13.11 Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes

3) Random Fertilization The fusion of gametes Will produce a zygote with any of about 64 trillion diploid combinations

Evolutionary Significance of Genetic Variation Within Populations Is the raw material for evolution by natural selection