Meiosis & Sexual Reproduction 2007-2008

Aaaargh! I’m seeing double! Cell division Mitosis produce cells with same information ____________________________ exact copies _______________________ same amount of DNA ___________________________ same genetic information Aaaargh! I’m seeing double!

The Human Life Cycle Gametes vs. Somatic Cells ____________: Sperm or ovary cells Somatic Cells: Any other cell besides sperm or ova (muscle, nerve, etc.) ________________________________________________ Human gametes contain 23 chromosomes

+ 46 46 92 How about the rest of us? What if a complex multicellular organism (like us) wants to reproduce? __________________________ Do we make egg & sperm by mitosis? What if we did, then…. 46 + 46 92 egg sperm zygote Doesn’t work!

Human female karyotype 46 chromosomes 23 pairs

Human male karyotype 46 chromosomes 23 pairs

Homologous chromosomes Paired chromosomes both chromosomes of a pair carry “matching” genes control same inherited characters _____________________________ single stranded homologous chromosomes diploid 2n 2n = 4 double stranded homologous chromosomes

Chromosomes A chromosome is a long coiled-up strand of DNA Humans have 23 pairs of homologous chromosomes in each somatic cell Homologous Chromosomes: Chromosomes that have the same length, centromere position, and staining pattern Carry genes controlling the same inherited characteristics (ie. Eye color, attached ear lobes) One chromosome of each pair comes from each parent Only exception… _______________________________________ Female sex chromosomes (XX) -homologous

How do we make sperm & eggs? Must reduce 46 chromosomes  23 _________________________________ 23 46 23 46 egg 23 meiosis 46 23 zygote fertilization sperm gametes

Meiosis Big Picture: Replication of chromosomes (DNA) followed by two consecutive cell divisions Results in 4 different daughter cells, each with only half as many chromosomes as the parent (haploid)

Steps of meiosis Meiosis 1 Meiosis 2 interphase prophase 1 metaphase 1 anaphase 1 telophase 1 Meiosis 2 prophase 2 metaphase 2 anaphase 2 telophase 2 1st division of meiosis separates homologous pairs (2n  1n) “reduction division” 2nd division of meiosis separates sister chromatids (1n  1n) * just like mitosis *

Meiosis: production of gametes Alternating processes, alternating stages chromosome number must be reduced diploid  haploid _______________ humans: 46  23 meiosis reduces chromosome number makes gametes fertilization restores chromosome number haploid  diploid ____________

Meiosis Reduction Division special cell division in sexually reproducing organisms _______________ diploid  haploid “half” ________________ sperm, eggs Warning: meiosis evolved from mitosis, so stages & “machinery” are similar but the processes are radically different. Do not confuse the two!

Preparing for meiosis 1st step of meiosis _________________ Why bother? meiosis evolved after mitosis convenient to use “machinery” of mitosis DNA replicated in S phase of interphase of MEIOSIS (just like in mitosis) 2n = 6 single stranded 2n = 6 double stranded M1 prophase

Meiosis I starts with diploid cell with duplicated chromosomes ends with separation of homologous chromosomes in 2 daughter cells which are each haploid but with duplicated chromosomes (reductional division)

Meiosis I: Separating Homologous Chromosomes Prophase I: Chromosomes begin to condense (you can see them!) _______________: Homologous chromosomes come together (4 sister chromatids total) Chiasmata Chromatids cross over at various places, known as chiasmata An exchange of genetic information occurs _______________________________ Spindle microtubules begin to form

What are the advantages of sexual reproduction? Crossing over What are the advantages of sexual reproduction? 3 steps ________________ breakage of DNA New combinations of traits

Meiosis I: Separating Homologous Chromosomes Metaphase I: Chromosomes are arranged on the metaphase plate in homologous pairs

Meiosis I: Separating Homologous Chromosomes ___________: The spindle fibers move the chromosomes toward the poles Each homologous chromosome of a tetrad goes to opposite poles

Meiosis I: Separating Homologous Chromosomes Telophase I: The chromosomes reach the poles of the cell _______________ Animals: cleavage furrow Plants: cell plate

Meiosis II: Separating Sister Chromatids Prophase II: Centrioles duplicate Chromosomes move towards middle Metaphase II: _______________________________________________

Meiosis II: Separating Sister Chromatids Anaphase II: Centromeres of sister chromatids separate Sister chromatids move toward opposite poles of the cell Telophase II/Cytokinesis: Nuclei form at opposite poles of the cell Cytokinesis occurs ______________________________________________

The value of sexual reproduction Sexual reproduction introduces genetic variation genetic recombination during meiosis independent assortment of chromosomes random alignment of homologous chromosomes in Meiosis 1 crossing over random fertilization which sperm fertilizes which egg? Driving evolution variation for natural selection metaphase1

Meiosis 1

Meiosis 2

Mitosis vs. Meiosis

Variation from genetic recombination Independent assortment of chromosomes ________________________________ gametes of offspring do not have same combination of genes as gametes from parents random assortment in humans produces 223 (8,388,608) different combinations in gametes offspring from Mom from Dad new gametes made by offspring