Meiosis Sexual Reproduction AP Biology 2007 2008 Cell

Meiosis & Sexual Reproduction AP Biology 2007 -2008

Cell division / Asexual reproduction § Mitosis u produce cells with same information § identical daughter cells u exact copies § clones u same amount of DNA § same number of chromosomes § same genetic information § What is Binary Fission? AP Biology Aaaargh! I’m seeing double!

Okay then…. ? § Why can’t we just do mitosis to reproduce? § Why produce sperm and egg and even go through § the hassle of dating? If we are going to do it can we make egg & sperm by mitosis? No! What if we did, then…. 46 egg AP Biology + 46 92 sperm zygote Doesn’t work!

Sexual reproduction creates variability Sexual reproduction allows us to maintain both genetic similarity & differences. Jonas Brothers AP Biology Baldwin brothers Martin & Charlie Sheen, Emilio Estevez

The value of sexual reproduction § Sexual reproduction introduces genetic variation u genetic recombination § independent assortment of chromosomes w random alignment of homologous chromosomes in Metaphase 1 u crossing over § mixing of alleles across homologous chromosomes u random fertilization § which sperm fertilizes which egg? § Driving evolution u providing variation for natural selection metaphase 1 AP Biology

Human female karyotype 46 chromosomes 23 pairs AP Biology

Human male karyotype 46 chromosomes 23 pairs AP Biology

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

Meiosis: production of gametes u chromosome number must be reduced § diploid haploid § 2 n n haploid w humans: 46 23 § meiosis reduces chromosome number § makes gametes u fertilization restores chromosome number § haploid diploid § n 2 n diploid AP Biology

Double division of meiosis DNA replication 1 st division of meiosis separates homologous pairs AP Biology 2 nd division of meiosis separates sister chromatids Meiosis 1 Meiosis 2

Meiosis 1 § 1 st division of meiosis 2 n = 4 single stranded separates homologous pairs prophase 1 2 n = 4 double stranded metaphase 1 2 n = 4 double stranded synapsis tetrad reduction telophase 1 AP Biology 1 n = 2 double stranded

Trading pieces of DNA § Crossing over u during Prophase 1, sister chromatids intertwine § homologous pairs swap pieces of chromosome w DNA breaks & re-attaches synapsis tetrad AP Biology prophase 1

Meiosis 2 § 2 nd division of meiosis 1 n = 2 double stranded separates sister chromatids prophase 2 What does this division look like? 1 n = 2 single stranded AP Biology 1 n = 2 double stranded metaphase 2 4 telophase 2

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

Mitosis vs. Meiosis § Mitosis u u u AP Biology 1 division daughter cells genetically identical to parent cell produces 2 cells 2 n produces cells for growth & repair no crossing over § Meiosis u 2 divisions daughter cells genetically different from parent produces 4 cells 2 n 1 n produces gametes u crossing over u u

Putting it all together… meiosis fertilization mitosis + development gametes 46 meiosis 46 23 egg 23 23 23 zygote fertilization AP Biology sperm 46 46 46 mitosis development

Sperm production Epididymis Testis Coiled seminiferous tubules germ cell (diploid) primary spermatocyte (diploid) MEIOSIS I secondary spermatocytes (haploid) Vas deferens spermatids (haploid) spermatozoa § Spermatogenesis continuous & prolific process Cross-section of seminiferous tubule u each ejaculation = 100 -600 million sperm AP Biology u MEIOSIS II

Egg production: Oogenesis § Begins in the ovaries of the § § female fetus before birth u Pauses during first meiotic division Final development occurs in the ovaries of the adult female u Each month one egg matures as cued by hormones u Completes the first meiotic division and starts the second meiotic division Last bit of meiosis is finalized at time of fertilization AP Biology

Putting all your egg in one basket! Oogenesis germinal cell (diploid) primary follicles fallopian tube fertilization primary oocyte (diploid) MEIOSIS I secondary oocyte (haploid) first polar body MEIOSIS II after fertilization second polar body AP Biology ovum (haploid) developing follicle mature follicle with secondary oocyte ruptured follicle (ovulation) corpus luteum

sperm vs. egg production § Similarities Both produce haploid cells by meiosis u Both take place in the gonads u both are controlled by hormones § Differences u Spermatogenesis produces 4 sperm each time while oogenesis produces only 1 egg u Formation of mature sperm continually occurs while eggs only mature once a month (on average) u Sperm formation never stops, egg formation ends at menopause u Sperm can be released at anytime while eggs AP Biology are released only once a month u

Differences across kingdoms § Not all organisms use haploid & diploid stages in same way u u which one is dominant (2 n or n) differs but still alternate between haploid & diploid § must for sexual reproduction AP Biology

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