Meiosis & Sexual Reproduction
Cell division / Asexual reproduction! Mitosis " produce cells with same information! identical daughter cells " exact copies! clones " same amount of DNA! same number of chromosomes! same genetic information Aaaargh! I m seeing double!
Asexual reproduction:! Single-celled eukaryotes " yeast (fungi) " Protists! Paramecium! Amoeba! Simple multicellular eukaryotes " Hydra budding budding What are the disadvantages of asexual reproduction? What are the advantages?
How about the rest of us?! What if a complex multicellular organism (like us) wants to reproduce? " joining of egg + sperm! Do we make egg & sperm by mitosis? What if we did, then. No! 46 + 46 92 egg sperm zygote Doesn t work!
Preparing a Karyotype: What kind of info. can be obtained? Application Technique Pair of homologous duplicated chromosomes Centromere 5 µm Sister chromatids Metaphase chromosome Why is it more practical to prepare karyotypes by viewing somatic diploid cells rather than haploid gametes? Both sets of chromosomes, which are present in somatic diploid cells, need to be examined.
Describing chromosomes: the anatomy 2n = 6 Sister chromatids of one duplicated chromosome Maternal set of chromosomes (n = 3) Paternal set of chromosomes (n = 3)! In a cell in which DNA synthesis has occurred, each chromosome is replicated.! Each replicated chromosome consists of two identical sister chromatids. Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)
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! homologous = same information diploid 2n 2n = 4 single stranded homologous chromosomes double stranded homologous chromosomes
How do we make sperm & eggs?! Must reduce 46 chromosomes 23 " must reduce the number of chromosomes by half 46 46 23 egg 46 meiosis 23 sperm gametes 23 23 fertilization zygote
Meiosis: production of gametes! Alternating stages " chromosome number must be reduced! diploid haploid! 2n n # humans: 46 23! meiosis reduces chromosome number! makes gametes " fertilization restores chromosome number! haploid diploid! n 2n haploid diploid
Sexual reproduction lifecycle:! 2 copies! diploid! 2n! 1 copy! haploid! 1n fertilization In the next generation We re mixing things up here! A good thing? meiosis! 1 copy! haploid! 1n gametes gametes
Meiosis:! Reduction Division " special cell division for sexual reproduction " reduce 2n 1n " diploid haploid! two half " makes gametes! sperm, eggs Warning: meiosis evolved from mitosis, so stages & machinery are similar but the processes are radically different. Do not confuse the two!
Overview of meiosis: 2n = 4 I.P.M.A.T.P.M.A.T interphase 1 prophase 1 metaphase 1 anaphase 1 n = 2 n = 2 prophase 2 metaphase 2 anaphase 2 telophase 2 n = 2 telophase 1
Double division of meiosis: DNA replication Repeat I can t after hear you! me! 1st division of meiosis separates homologous pairs 2nd division of meiosis separates sister chromatids Meiosis 1 Meiosis 2
Preparing for meiosis:! 1st step of meiosis " Duplication of DNA " 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) M1 prophase 2n = 6 single stranded 2n = 6 double stranded
Meiosis 1:! 1st division of meiosis separates homologous pairs prophase 1 2n = 4 single stranded 2n = 4 double stranded synapsis tetrad I can t hear you! Repeat after me! telophase 1 1n = 2 double stranded metaphase 1 2n = 4 double stranded reduction
Independent Assortment of homologous chromosomes in Metaphase I leads to genetic variation in offspring Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4
Meiosis 2:! 2nd division of meiosis separates sister chromatids 1n = 2 double stranded prophase 2 What does this division look like? 1n = 2 single stranded 1n = 2 double stranded metaphase 2 4 telophase 2
Steps of meiosis reduction division! Meiosis 1 " 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 2nd division of meiosis separates sister chromatids (1n 1n) division * just like mitosis *
Meiosis 1 & 2
Trading pieces of DNA:! Crossing over synapsis " during Prophase 1, sister chromatids intertwine! homologous pairs swap pieces of chromosome # DNA breaks & re-attaches prophase 1 tetrad
Crossing over: during Prophase 1! 3 steps " cross over " breakage of DNA What are the advantages of crossing over in sexual reproduction? " re-fusing of DNA! New combinations of traits
Various Hypothesis on Sexual Reproduction: Sexual reproduction is advantageous to species that benefit from genetic variability. However, since evolution occurs because of changes in an individual's DNA, crossing over and chromosome segregation is likely to result in progeny that are less well-adapted than their parents. On the other hand, asexual reproduction ensures the production of progeny as fit as the parent since they are identical to the parent. Remember the adage, if it's not broken, don't fix it. There are several hypotheses regarding the evolution of sexual reproduction. 1) One is associated with repairing double-stranded DNA breaks induced by radiation or chemicals. 2) The contagion hypothesis suggests that sex arose from infection by mobile genetic elements. 3)The Red Queen hypothesis theorizes that sex is needed to store certain recessive alleles in case they are needed in the future. Along similar lines, eukaryotic cells build up large numbers of harmful mutations. 4) According to Miller's Rachet hypothesis, Sex, as explained by, may simply be a way to reduce these mutations. The whole truth is likely a combination of these factors. Regardless of how and why, the great diversity of vertebrates and higher plants and their ability to adapt to the highly varied habitats is indeed a result of their sexual reproduction.
Mitosis vs. Meiosis
Mitosis vs. Meiosis! Mitosis " 1 division " daughter cells genetically identical to parent cell " produces 2 cells " 2n 2n " produces cells for growth & repair " no crossing over! Meiosis " 2 divisions " daughter cells genetically different from parent " produces 4 cells " 2n 1n " produces gametes " crossing over
Putting it all together meiosis fertilization mitosis + development gametes 46 23 46 23 meiosis 46 egg sperm 23 23 46 46 46 46 46 46 46 46 zygote fertilization mitosis development
The value of sexual reproduction:! Sexual reproduction introduces genetic variation " genetic recombination! independent assortment of chromosomes # random alignment of homologous chromosomes in Metaphase 1 " crossing over " mixing of alleles across homologous chromosomes during Prophase I " random fertilization! which sperm fertilizes which egg?! Driving evolution " providing variation for natural selection metaphase1
Variation from genetic recombination:! Independent assortment of chromosomes " meiosis introduces genetic variation " gametes of offspring do not have same combination of genes as gametes from parents! random assortment in humans produces 2 23 (8,388,608) different combinations in gametes from Mom from Dad offspring new gametes made by offspring
Variation from crossing over:! Crossing over creates completely new combinations of traits on each chromosome " creates an infinite variety in gametes
Variation from random fertilization:! Sperm + Egg =? " any 2 parents will produce a zygote with over 70 trillion (2 23 x 2 23 ) possible diploid combinations
Sexual reproduction creates variability: Sexual reproduction allows us to maintain both genetic similarity & differences. Jonas Brothers Baldwin brothers Martin & Charlie Sheen, Emilio Estevez
Sperm production: Epididymis Testis Vas deferens Coiled seminiferous tubules germ cell (diploid) primary spermatocyte (diploid) secondary spermatocytes (haploid) spermatids (haploid) MEIOSIS I MEIOSIS II spermatozoa! Spermatogenesis Cross-section of seminiferous tubule " continuous & prolific process " each ejaculation = 100-600 million sperm
Egg production:! Oogenesis " eggs in ovaries halted before Anaphase 1 " Meiosis 1 completed during maturation " Meiosis 2 completed after fertilization " 1 egg + 2 polar bodies unequal divisions Meiosis 1 completed during egg maturation What is the advantage of this development system? ovulation Meiosis 2 completed triggered by fertilization
Oogenesis: Putting all your egg in one basket! germinal cell (diploid) primary follicles fallopian tube fertilization MEIOSIS I first polar body MEIOSIS II after fertilization second polar body primary oocyte (diploid) secondary oocyte (haploid) ovum (haploid) developing follicle mature follicle with secondary oocyte ruptured follicle (ovulation) corpus luteum
Differences across kingdoms:! Not all organisms use haploid & diploid stages in same way. Only diploid cells can undergo meiosis. Why? " which one is dominant (2n or n) differs " but still alternate between haploid & diploid! must for sexual reproduction
Any Questions?? What are the DISadvantages of sexual reproduction? 2007-2008