Download MEIOSIS AND SEXUAL LIFE CYCLES

Meiosis and Sexual Life
Cycle
Chapter 13
• Heredity (inheritance) transmission of traits from 1
generation to next.
• Variation - siblings differ from 1
another.
• Genetics - study of heredity and
variation.
http://www.mssm.edu/genetics/fabry/images/figure1b.gif
• Parents give offspring coded
information - genes.
• Genes - segments of DNA.
• 4 nucleotides found in DNA create
specific sequences determine
traits.
• DNA in sperm and egg fuse
together to create variation.
http://library.thinkquest.org/19037/media/basics/gene.GIF
• Each chromosome has thousands of
genes - located at specific spot locus (loci).
http://www.cybered.net/library/Teaching_Resources/Biology/Evolution/Image_Gallery/Evolution-Loci.jpg
• Asexual reproduction - 1 parent
passes all genetic information on to
offspring.
• Example: Hydra (eukaryotic)
reproduce by budding cells
produced by mitosis.
• Sexual reproduction produces
variation - combination of genes
from parents unique.
• Siblings resemble each other
because of similar genes.
• Humans, somatic (non-sex) cells 46 chromosomes.
• Each chromosome distinguished by
size, position of centromere, by
pattern of staining with certain
dyes.
• Karyotype - picture of 23 pairs of
chromosomes with centromeres and
sizes.
• Chromosomes homologous - they
have pair that matches them.
• Sex chromosomes not homologous in
male (X and Y)
• Females homologous (2 X’s).
• Other 22 pairs autosomes (non-sex
chromosomes)
• 1 chromosome of pair inherited
from each parent.
http://www.brooklyn.cuny.edu/bc/ahp/LAD/C9/graphics/C9_homologous_2.GIF
• Each egg and sperm (gametes) have
22 autosomes, 1 sex chromosome.
• Haploid - half the # of
chromosomes.
• Cells fuse (syngamy) - fertilization.
• Fertilized egg (zygote) has 2
haploid sets of chromosomes with
genes from maternal and paternal
family lines.
http://faculty.sulross.edu/ericsson/img0067b.jpg
• When zygote has all chromosomes
(46), - diploid.
• Humans - diploid # of chromosomes
46 (2n = 46).
• Gametes (develop in gonads) and
are produced through meiosis chromosome # halved.
• Fertilization and meiosis alternate
in sexual life cycles.
• Meiosis and fertilization timing
varies among species.
• Fungi, some protists have 2 life
cycles.
• Zygote starts off diploid, then
divides to haploid.
• Haploid adult goes through mitosis.
Human life cycle
11/28
• Plants - alternation of generations.
• Includes haploid (gametophyte) and
diploid (sporophyte) multicellular
stages.
• Meiosis by sporophyte produces
haploid spores - develop by mitosis
into gametophyte.
• Meiosis - 2 consecutive cell
divisions (meiosis I, meiosis II) result in 4 daughter cells.
• Each daughter cell - only ½ as many
chromosomes as parent cell.
• Meiosis reduces chromosome # by
copying chromosomes once, but
dividing twice.
Meiosis I
• Stages are prophase, metaphase,
anaphase, telophase.
• Interphase (preceding prophase) chromosomes replicate to form
sister chromatids.
• Single centrosome replicated.
• Prophase I - chromosomes
condense, homologous chromosomes
pair up - form tetrads.
• Synapsis - proteins attach
homologous chromosomes tightly
together.
• Chromatids of homologous
chromosomes cross (chiasmata),
segments of chromosomes traded.
• Spindle forms from each
centrosome.
• Metaphase I - tetrads arranged at
metaphase plate.
• Anaphase I - homologous
chromosomes separate, pulled
toward opposite poles.
• Telophase I - movement of
homologous chromosomes continues
until haploid set at each pole.
• Each chromosome consists of linked
sister chromatids.
• Cytokinesis - separates cytoplasm.
• Prophase II - spindle apparatus
forms, attaches to kinetochores of
each sister chromatid, moves them
around.
• Metaphase II - sister chromatids
arranged at metaphase plate.
• Anaphase II - centomeres of sister
chromatids separate; travel toward
opposite poles.
• Telophase II - separated sister
chromatids arrive at opposite poles;
nuclei form around chromatids.
• Cytokinesis - cytoplasm separates;
now 4 daughter cells.
Differences between mitosis and
meiosis
• Chromosome # reduced by ½ in
meiosis, not in mitosis.
• Mitosis produces 2 genetically
identical cells; meiosis produces 4
unique ones.
http://ghr.nlm.nih.gov/handbook/illustrations/mitosismeiosis
Other differences
• 1Crossing over: During prophase I,
homologous chromosomes pair up
(synapsis).
• At X-shaped regions (chiasmata)
sections of nonsister chromatids
exchanged.
http://www.synapses.co.uk/genetics/crosso1.gif
• 2Metaphase I homologous pairs of
chromosomes, not individual
chromosomes, aligned along
metaphase plate.
• 3Anaphase I - homologous
chromosomes, (not sister
chromatids) separate, carried to
opposite poles of cell.
Contributions to Variation
• 1Independent assortment - tetrads
arrange themselves randomly on
metaphase plate.
• Each homologous pair of
chromosomes positioned
independently of other pairs at
metaphase I.
• 2Crossing over - homologous
portions of 2 nonsister chromatids
trade places.
• Crossing over begins early in
prophase I as homologous
chromosomes pair up gene by gene.
• 3Random fertilization - any sperm
can fertilize any egg.
• Each egg sperm is 1 of 8 billion gene
combinations - 70 trillion
possibilities of combinations in
zygote (doesn’t include crossing
over)
http://www.itech.pjc.edu/fduncan/bsc1094/ap2c29ppt_files/slide0007_image008.jpg