MEIOSIS
Features of Reproductive Systems
Humans are diploid (2n) and reproduce sexually by fertilization through the union of haploid (1n) gametes (sex cells)
Human diploid chromosome number: 46 chromosomes
22 pairs of homologous chromosomes
1 homolog from mother
1 homolog from father
Homologs do not have exactly the same sequence
1 pair of sex chromosomes
XX: Female
XY: Male
Gametes are sperm and egg from male and female
Haploid
Fertilization results in fusion of gamete nuclei to form diploid zygote to give 22 pairs of chromosomes
Zygote divides by mitosis to form (somatic) body cells and grow into the adult organism
Reproductive systems are blind-ended tubes with gametes produced at the proximal end and products released at the distal end
Produced close to the body, sperm and egg
Excreted at the tip/outside
Gonads: proximal
Excurrent path: Between
Penis/Vagina: DIstal
Reproductive organs are functionally homologous (same) for the male and female but structurally different
Gonads are organs that produce gametes by meiosis
Male: Testes
Female: Ovary
Gonads produce and secrete sex hormones (steroids) made from cholesterol to facilitate gamete production and sexually dimorphic (same species different parts or looks) features
Testis: Testosterone
Ovary: Estrogen and progesterone
Excurrent paths (ducts) for transport of gametes
Male path: Epididymis→vas deferens→ejaculatory duct→urethra
Female path: Uterus→Vagina
Glands for exocrine secretions that FACILITATE fertilization
MALE: Seminal vesicle, prostate, bulbourethral (Cowper’s) glands
FEMALE: Uterus, bulbourethral (Bartholin’s) gland
Erectile Tissue involving blood hydrostatic pressure (ERECTION)
Male: Penis
Female: Clitoris
Female reproductive system is specialized for nutrition of developing zygote and infant
Uterus: Zygote to infant development via placenta
Mammary gland: Infant nutrition (BREAST FEEDING)
Meiosis
eggs/sperm in prophase I
Mitosis and meiosis are processes that replicate cells
Mitosis is cellular replication/duplication
Mother cell divides to give two daughter cells with exactly the same genome
Daughter: Exactly the same DNA sequence as parent cell
Meiosis is reductive cell division
Mother cell divides to give 4 daughter cells that have similar genomes
Not exactly alike
Gives genetic diversity to gametes
Gametes have different combinations of genes on chromosomes compared to mother cell involving
Homologous recombination (crossing over) between homologous chromosomes during prophase I
Random segregation of maternal and paternal chromosomes during anaphase I
PMAT→PP1M1A1T1
PRIOR TO MITOSIS: CELL CYCLE
Cell cycle is involved in producing a large number of gametes
Involves three major phases:
Interphase: when chromosomes are not visible by light microscopy but stuff is going on
Karyokinesis: Packing up and separation of chromosomes
Splitting nucleus
Cytokinesis: Splitting cytoplasm to give two daughter cells each containing a nucleus
*Include checkpoints to see if there is enough growth factors to allow for cell division
G1: Cell Growth of cytoplasm
S-Phase: DNA replication
Make sister chromatid of chromosome
Sisters get linked together
Sisters have the exact same DNA sequence
G2: Prepare for cell division (karyokinesis and cytokinesis)
Mitosis: PMAT
Cytokinesis
Gametogenic cells develop early in the embryo and can proliferate by mitosis prior to entering meiosis
Male: Proliferate after puberty and enter meiosis
Female: proliferate during embryonic development, begin meiosis, arrest in meiosis 1 and complete meiosis after puberty
Gametogenic (-gonia) cells/germ cells: Diploid cells that produce gamates
Males: Spermatogonia
Females: Oogonia
Humans produce hundreds of thousands of trillions of gamates
Females: Hundreds of thousands but functionally hundreds
Limited number released in lifetime
Males: Trillions
Produced whole life
Meiosis
Prophase I
Chromosomes (sisters) compacted
Homologs pair up to form the tetrad
Maternal sisters pair up with paternal sisters for each chromosome
Crossing over: A sister of each homolog swap two or three regions of their chromosomes
Homologous recombination that unlinks genes on a chromosome
Gives a unique combination of genes from the maternal and paternal homologs on the recombinant chromosomes
Chiasma: Regions where recombination occurred remain attached
Separate during metaphase I
Very complex processes taking a lot of time hence the longest duration of meiosis
Most female eggs are in this stage for 15-45 years
Metaphase I
Microtubules attach to each homolog at the centromere
Random attachment relative to aster later results in random segregation of maternal and paternal homologs for each chromosome
Tetrads align at the midline of the cell, SPINDLE ATTACHES
Anaphase I
Homologs separated at the chiasma (NOT CENTROMERES)
Separation of homologs reduces ploidy from diploid to haploid
2n→1n
Future cell will either have the maternal or paternal version of each chromosome
1x DNA and haploid chromosome number (46→23)
Telophase I
Homologs pulled to poles and reform nucleus
Cytokinesis
Separates cytoplasm
INTERKINESIS: BETWEEN MEIOSIS I AND MEIOSIS II
Centrioles replicate but not DNA
Meiosis II
Involves karyokinesis and cytokinesis to produce two haploid cells with ½ the DNA and chromosomes
Prophase II
Condensation of chromatin to form visible chromosomes
Metaphase II
Microtubules attach to each sister of each chromosome
Sisters for each chromosome line-up along the midline
Spindle attaches
Anaphase II
Sisters separated at the centromere
Telophase II
Nuclei form at poles
Cytokinesis to separate the cells
Molecular Aspects about Meiosis
Meiosis is a dorm of highly regulated double stranded DNA cleavage of one sister of one homolog followed by repair of using one sister of the other homolog
Double stranded DNA repair involves removing some of the DNA sequence at the cut site
Need other homolog’s sequence to use for filling in
Repair is followed by the separation of the Holliday junction and the axis of separation determines whether there is a crossing over event or a gene replacement
Crossing over results in a recombinant chromosome where 1 part is from one homolog and the other from the other
Chromosome now has some maternal and some paternal genes
Of the four sisters, two are recombinant (mix of maternal and paternal chromosome regions), and two are not (just maternal or paternal)
Locations of crossing over have evolved such that they are same to give regions of the chromosome that remain intact
Haplotype regions
Sites of cleavage are called recombination hot-spots
Haplotype regions are used to determine ancestry and individual identification
Recombination regions are in non-protein coding regions of the genome
Recombination does result in repair errors giving mutation in these regions
Recombination sites and frequency used to determine proximity of genes
Linkage analysis and linkage disequilibrium in pedigrees and populations
Hot where recombination, cold where low recombination
Errors during Meiosis
Errors result from failure to separate homologs during meiosis I or sister chromatids during meiosis II
Called nondisjunction errors: did not disjoin
Most found in the population are sex chromosome aneuploidies with only am aneuploidy in chromosome 21 being compatible with life
Gene (chromosome) dosage in autosomes is important
Under diagnosed because phenotypic effects are usually minor
Turner Syndrome
Only found with XO, females
Trisomy 21: Down’s Syndrome
XXXn: Trisomy or higher order X in females
Tall females
XXY: Klinefelter Syndrome
High luteinizing hormone
Cryptorchidism: Low testosterone due to testicles not descending
XYY: Jacobs Syndrome
XX Mosaic: White spot in hair