Sexual Reproduction
HeredityHeredity - passing down of characteristics (genesgenes)
AllelesAlleles- different forms of the same gene
Sexual Reproduction
Why sexual reproduction? shuffles alleles;
new combinationsprovides genetic
variation in species
Meiosis Type of cell divisioncell division
One germ* cell makes 4 gametes (egg/sperm) with ½ the # of chromosomes.
Occurs only in germ* cells of gonads ○ testes /ovaries
Occurs in flowers ○ ovary and anther
*“Germ” here doesn’t mean bacteria. It’s a term used for the type cells that gives rise to gametes in organisms.
While females are born with all the eggs they will ever have (around 400,000), only about 400 will ever mature and ovulate between puberty and menopause.
This shows how only one single sperm gets to penetrate the egg, releasing its nucleus of 23 chromosomes to merge with the nucleus of the egg and its 23 chromosomes.
Homologous Homologous ChromosomesChromosomes
Pair of chrom. similar in shape , size, and types of genes. Each locus (location of the gene) in same position on chrom.
Humans have 23 pairs of homologueshomologues Housefly – 6 prs Housefly – 6 prs Chicken – 39 prs Chicken – 39 prs Apple – 17 prsApple – 17 prs Dog – 39 prsDog – 39 prs Cat – 19 prs Cat – 19 prs
This is a karyotype (an image of an organism’s chromosomes)
This is a karyotype of a normal human male
Chromosome numbers: However many “types” of chrom. an organism has,
that number is the “n” number of chrom. it has.
Body cells Egg/Sperm cells
Called: Somatic cells Gametes
Term for chrom. #
Diploid (2 sets of chrom.)
Haploid(1 set of chrom.)
“n” number 2n 1n
For Humans: 46 chrom. 23 chrom.
Homologous Homologous ChromosomesChromosomes
Paternal(from Dad)
Maternal (from Mom)
eye color locus
eye color locus
hair color locus
hair color locus
Prophase IProphase I
Longest and most complex phase (90% of meiosis). DNA condenses into chromosomes. SynapsisSynapsis - - a process: when homologous chromhomologous chrom. .
come together, pair up, form a tetradtetrad.
Prophase IProphase I - - SynapsisSynapsis
Nonsister chromatids
sister chromatids sister chromatidsTetrad
Prophase I -Prophase I - Crossing Crossing OverOver
Crossing over may occur in the tetrad:between nonsister chromatids, ends break and reattach
Crossing Over - Provides Crossing Over - Provides
VariationVariation
nonsister chromatids
Chiasma: site of crossing over
variation
Tetrad
Metaphase IMetaphase I
Shortest phase; paired homologues align. INDEPENDENT ASSORTMENTINDEPENDENT ASSORTMENT occurs
pairs of homologues line up independently of other pairs’ orientation toward the poles -- random. Adds variation.
Anaphase IAnaphase I Homologous chromosomes separate towards
the poles (Tetrads separate)
Sister chromatids Sister chromatids remain attached
Telophase ITelophase I Each pole now has haploid set of chromosomes Each pole now has haploid set of chromosomes
(however – still doubled).(however – still doubled). CytokinesisCytokinesis occurs: two haploid daughter cells
formed.
Meiosis IIMeiosis II No interphase II No interphase II
( no more DNA replicationDNA replication) Remember: Meiosis II Remember: Meiosis II is similar to mitosismitosis
Prophase II / Metaphase II
Anaphase II
Telophase IITelophase II Same as telophasetelophase in mitosismitosis. Nuclei form. CytokinesisCytokinesis occurs (2nd time). Four haploid daughter cells produced (chromosomes now
back to single condition).gametes ~ sperm or egg; ovule or pollen grain
Gamete Formation in Animals Diff. bet. male and female gametes.
Male: spermatogenesisMale: spermatogenesis all 4 develop into sperm cells.
Female: oogenesisFemale: oogenesiscytokinesis is uneven. most cytoplasm goes to 1 of the 4 “eggs”
(forms 1 large egg cell) 3 other cells are small “polar bodies” which die
SpermatogenesiSpermatogenesiss
2n = 46
humangerm cell in testes
diploid (2n)
n=23Still doubled chromosomes
n=23Still doubled chromosomes
meiosis I
n=23
n=23
n=23
n=23
sperm
haploid (n)
meiosis II
Oogenesis
2n = 46
human germ cellin ovary
diploid (2n)
n=23Still doubled
n=23Still doubled
meiosis I
23
23
23
Ovumn=23
Polar Bodies
meiosis II
Mitosis vs Meiosis Mitosis
Body (somatic cells) 2 daughter cells made
(identical) Each w/ same # & kind
of chrom. as parent cell 1 division process 1 cytokinesis No synapsis or
crossing over Are diploid (2n)
Meiosis Germ cells of gonads 4 gamete cells made
(all different) Each w/ ½ chrom. # as
parent cell 2 divisions 2 cytokineses events Synapsis &
crossing over occurs in Prophase 1
Are haploid (n)
Nondisjunction
When the tetrad (in Anaphase I) or the sister chromatids (in Anaphase II) do not separate, creating an abnormal # of chrom. to occur in the gametes.
Lethal most of the time