Meiosis & Sexual Reproduction 2007-2008

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 What is Binary Fission? Aaaargh! I’m seeing double!

+ 46 46 92 Okay then….? No! What if we did, then…. Doesn’t work! 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 + 46 92 egg sperm zygote Doesn’t work!

Types of Reproduction ASEXUAL Produces clones (genetically identical) Single parent Little variation in population - only through mutations Fast and energy efficient Eg. budding, binary fission SEXUAL Meiosis produces gametes (sex cells) 2 parents: male/female Lots of variation/diversity Slower and energy consumptive Eg. humans, trees

Chromosomes Somatic (body) cell: 2n = 46 chromosomes Each pair of homologous chromosomes includes 1 chromosome from each parent Autosomes: 22 pairs of chromosomes that do not determine sex Sex chromosomes: X and Y Females: XX Males: XY Gametes (n=23): 22 autosomes + 1 sex chromosome Egg: 22 + X Sperm: 22 + X **or** 22 + Y

Sexual reproduction creates variability Sexual reproduction allows us to maintain both genetic similarity & differences. Jonas Brothers Baldwin brothers Martin & Charlie Sheen, Emilio Estevez any 2 parents will produce a zygote with over 70 trillion (223 x 223) possible diploid combinations

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 random fertilization which sperm fertilizes which egg? Driving evolution providing variation for natural selection 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 metaphase1

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 single stranded homologous chromosomes diploid 2n 2n = 4 double stranded homologous chromosomes

Meiosis: production of gametes chromosome number must be reduced (reduction division) diploid  haploid 2n  n humans: 46  23 meiosis reduces chromosome number makes gametes fertilization restores chromosome number haploid  diploid n  2n haploid Warning: meiosis evolved from mitosis, so stages & “machinery” are similar but the processes are radically different. Do not confuse the two! diploid

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

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

Meiosis I (1st division) Interphase: chromosomes replicated Prophase I: Synapsis: homologous chromosomes pair up Tetrad = 4 sister chromatids Crossing over at the chiasmata Metaphase I: Tetrads line up Anaphase I: Pairs of homologous chromosomes separate (Sister chromatids still attached by centromere) Telophase I & Cytokinesis: Haploid set of chromosomes in each cell Each chromosome = 2 sister chromatids Some species: chromatin & nucleus reforms

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

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

Meiosis II (2nd division) = create gametes Prophase II: No interphase No crossing over Spindle forms Metaphase II: Chromosomes line up Anaphase II: Sister chromatids separate Telophase II: 4 haploid cells Nuclei reappear Each daughter cell genetically unique

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 *

Mitosis vs. Meiosis Mitosis Meiosis 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 46 46 46 46 46 23 meiosis 46 46 egg 46 46 23 zygote fertilization mitosis sperm development

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

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

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

Sources of Genetic Variation: Crossing Over Exchange genetic material Recombinant chromosomes

Sources of Genetic Variation: Independent Assortment of Chromosomes Random orientation of homologous pairs in Metaphase I

Sources of Genetic Variation: Random Fertilization Any sperm + Any egg 8 million X 8 million = 64 trillion combinations!

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

Life cycle: reproductive history of organism, from conception  production of own offspring Fertilization and meiosis alternate in sexual life cycles Meiosis: cell division that reduces # of chromosomes (2n  n), creates gametes Fertilization: combine gametes (sperm + egg) Fertilized egg = zygote (2n) Zygote divides by mitosis to make multicellular diploid organism

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

Human Life Cycle