Meiosis Is a special type of cell division in which certain somatic cells (containing the diploid number of chromosomes) divide into sex cells (gametes, containing the haploid number of chromosomes). It occurs only in the reproductive organs (the testes of the male and the ovaries of the female) in organisms that reproduce sexually. Each sex cell (sperm and ova) found is genetically unique.

Somatic cells : genetically identical Sex cells: not genetically identical

Meiosis – a somatic (body) cell in the reproductive organs divides into haploid sex cells Somatic Cell in testes/ovaries = oogenesis = spermatogenesis

22 autosomes in an egg plus a sex chromosome which is the x sex chromosome Autosomes – determines eye color, blood type, height, etc. Sex chromosome – determines the gender

22 autosomes in a sperm plus a x or y chromosome

xx in a zygote = female xy in a zygote = male

How can eggs only have the x sex chromosome? Because all female cells have 2 xx’s in all their nucleus

Sperm – smallest of all human cells Ovum – largest of all human cells Egg contributes more to the baby than the sperm does Egg – most of the nourishment - sugars, nutrients that is why it needs to be large

22 homologous (matching) pairs of autosomes

Chromosome Anomaly - Down Syndrome = 45 autosomes - extra chromosome in the egg - ex. 23 autosomes in an egg - a small percentage of eggs have an extra chromosome just as a small percentage of sperm have an extra chromosome

Per ejaculation – 360 million sperm Woman releases only 1 egg each month Woman – about 400 eggs in her lifetime

Karyotype – chromosome analysis

What about wrong number of sex chromosomes?

22 autosomes plus 2 xx in an egg plus sperm with 22 autosomes and an x sex chromosome – xxx female = mentally retarded

22 autosomes and xx chromosome and 22 autosomes from a sperm with an y chromosome = xxy = person looks male, mental retardation, sterility = pseudohermaphrodites xxy – klinefelter syndrome There are no hermaphrodites but there are pseudohermaphrodites = hormonal imbalance

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!

Okay then….? + 46 46 92 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!

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 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 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

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

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

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

Oogenesis MEIOSIS I MEIOSIS II primary follicles germinal cell 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

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