2What is Sexual Reproduction? Combines genetic material from two parents to produce a new individualCreates an increase in variation among the offspringPopulations can adapt to changing environmentsThe world is more enjoyable with variationCostly – it takes twice as many individuals to produce an offspring
3Homologous Chromosomes Genetic material (DNA) is found in chromosomesEach chromosome has a “twin” that:contains the same genescodes for the same traitshas the same shapeis the same sizeHas the same location of the centromereHas the same banding pattern when stainedOne chromosome came from the father, the other came from the motherThese twins are called homologous.
4Homologous Chromosomes Cells that have two of each chromosome are called Diploid (2n).This is the normal condition for most cells.Cells that have only one of each chromosome are called Haploid (n).This is half of the usual amount of DNA.Homologous Chromosomes
6Homologous Chromosomes Humans have 23 pairs of chromosomes – pairs of autosomes and one pair of sex chromosomesIn females they are all homologousIn males, the autosomes are homologous, but the sex chromosomes are notKaryotype – arranges chromosomes in homologous pairs
7Special Considerations: 1. If a sperm containing 46 chromosomes fused with an egg containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain?2. In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have?
8MeiosisPurpose:Divide the nucleus to reduce the number of chromosomes in halfEnsure that each new cell has a complete set of chromosomesCreate gametesAlso called reduction division
9Human Life Cycle Most of our cells are diploid (2n) Reproductive cells called gametes are haploid (n)They are produced in gonads (ovary and testes) from diploid gametocytesGametes
10Two Stages of Meiosis Meiosis I and Meiosis II A cell normally has two sets of chromosomes (2n)In meiosis I the homologous chromosomes are separatedIn meiosis II the sister chromatids are separatedFour cells are produced, each with one set of chromosomes (n).
11Meiosis I The phases of meiosis I are similar to mitosis: Prophase I, Metaphase I, Anaphase I, Telophase IThe formation of tetrads is called synapsisDyads are sister chromatids attached by the centromere
12Prophase 1 chromosomes condense homologs pair up forming tetrads (four chromatids are lined up)Nuclear envelope fragmentsCrossing-over can occur in this stage.
13What is Crossing Over? During synapsis of prophase I: Homologous chromosomes tangle and trade parts of their DNAVariation increasesGenes that are usually linked (on the same chromosome) can become separated
14Metaphase 1 Homologous chromosomes line up at the equatorial plate. Meiosis 1Metaphase 1Homologous chromosomes line up at the equatorial plate.
15Meiosis 1Anaphase 1Homologous pairs separate Sister chromatids remain together.
16Meiosis 1Telophase 1Two daughter cells are formed Each daughter contains only one chromosome of the homologous pair (haploid).
17Prophase II, Metaphase II, Anaphase II, Telophase II Meiosis IIAfter a phase called interkinesis (similar to G2), both cells formed in meiosis I go through phases againProphase II, Metaphase II, Anaphase II, Telophase IIInterkinesis
29Meiosis vs. Mitosis 2n 2n n n 2n Two cells genetically identical to parent cellFour cells not identical to parent
30Two daughter cells per cycle Four daughter cells per cycle MitosisMeiosisOne divisionTwo divisionsTwo daughter cells per cycleFour daughter cells per cycleDaughter cells genetically identicalDaughter cells genetically differentSame chromosome number as parentsChromosome number half that of parentsOccurs in somatic cellsOccurs in gametocytesThroughout life cycleCompleted after sexual maturityUsed in growth, repair, asexual reproductionSexual reproduction, new gene combinations