Chapter 2Chapter 2Mitosis & MeiosisMitosis & Meiosis
Cell TypesCell Types• Prokaryotes (bacteria)
– Single cell– No organelles, no nucleus (nucleoid –
circular DNA)
• Eukaryotes– Multicellular (generally)– Organelles, organized nucleus
Cheek cells
Bacteria
Cell OrganizationCell Organization• Nucleus• Rough/Smooth ER• Ribosomes• Cell membrane/cell wall • Cell coat (animal cells)• Cytoplasm, cytoskeleton• Mitochondria, chloroplasts, etc.
NucleusNucleus• Chromatin/chromosomes• Nucleolus (RNA Synthesis)
– Nucleolus Organization Region (NOR)• Sections of DNA that code for rRNA
DNA in the nucleusDNA in the nucleus
• Chromatin – loosely coiled DNA. DNA exists in this form until ready to divide.
ChromsomesChromsomes• Chromatin condenses before mitosis• Chromosome – tightly packed DNA, 2
“sister chromatids”
P ARM
Q ARM
KaryotypesKaryotypes• Karyotype: “chart” of all an
organism’s chromosomes
Homologous ChromosomesHomologous Chromosomes• Most organisms have 2 copies of
each chromosome (homologous)• Diploid (2n) – somatic cells • Haploid (n) – gametes. Haploid cells
contain only 1 copy of each chromosome
Homologous ChromosomesHomologous Chromosomes• 2 copies of each chromosome, 2
copies/version of each gene (ALLELE)
• Genes are located at identical sites on sister chromatids (LOCI)
Cell CycleCell Cycle
• A sequence of cell growth and division
• Numerous factors control when cells divide
• Chrom duplicate during INTERPHASE (90% of cell’s life)– G1 phase - cells grow and synthesize
biological molecules– S phase - DNA replication
– G2 phase - gap of time between S phase and mitosis (preparation for division)
• G0 phase
MitosisMitosis• Why do cells need to divide?• Zygote• How much time did you spend as a
single egg?
MitosisMitosis
• Purpose is to ensure the orderly distribution of chromosomes
• Four Stages:– Prophase– Metaphase– Anaphase– Telophase
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InterphaseInterphase• Chromosomes are extended &
uncoiled, forming chromatin• Centrioles duplicate
Prophase (early)Prophase (early)• Chromosomes condense, centrioles
divide and move apart
Prophase (middle)Prophase (middle)• Also called “prometaphase”• Chromosomes doubled, clearly
visible• Centrioles at poles, spindle fibers
form• Nucleolus dissapears• Nuclear membrane dissolves
MetaphaseMetaphase• Chromosomes line up in the middle
of the cell• Spindle fibers extend from the poles,
attached to the chromosomes
AnaphaseAnaphase• Centromeres split and sister
chromatids (now referred to as chromosomes) migrate to the poles
TelophaseTelophase• Two separate nuclei form• Cell returns to conditions similar to
interphase• Nuclear envelope reforms; nucleoli
reappear• Cytokinesis occurs
CytokinesisCytokinesis• In animals cells, a furrow develops
caused by filaments that encircle the equatorial region
• In plant cells, a cell plate forms originating from the Golgi complex
MeiosisMeiosis• Why must cells undergo meiosis?
– Reduce chromosome number– Production of gametes (gametogenesis)– Sexual reproduction involves the union
of gametes to form a zygote– Sexual reproduction results in greater
variation amongst offspring – offspring are not clones of their parents
How many combinations?How many combinations?• Number of combinations possible 2n,
where n is the haploid number of the organism
• If n = 3, there are 8 possible combos• Humans: n = 23, there are 223 ~ 8
million possible combinations of chromosomes
A few notes about chromosomes….A few notes about chromosomes….
• Autosomes: non-sex chromosomes – How many do humans have?
• Sex Chromosomes – Females: XX– Males: XY– Only small parts of these have the same
genes, most of their genes have no counterpart on the other chromosome
Meiosis – an overviewMeiosis – an overview• Meiosis reduces chromosome
number by copying the chromosomes once, but dividing twice
• The first division, meiosis I, separates homologous chromosomes
• The second, meiosis II, separates sister chromatids
Chromosomes separate
Chromatids separate
• Meiosis contains 2 stages, Meiosis I and Meiosis II
• Meiosis I and II each include prophase, metaphase, anaphase, and telophase
Meiosis I - Prophase IMeiosis I - Prophase I• Homologous chromosomes pair and
undergo synapsis (special proteins)• Synapsis is the association of four
chromatids (two copies of each homologous chromosome)
• The resulting complex is called a bivalent or tetrad– In humans, there are 23 tetrads and 92
chromatids in this phase
Prophase IProphase I• At several sites, the
chromatids of tetrads are crossed (chiasmata) and segments of chromosomes are traded (crossing over)
• Synaptonemal complex forms between members of the tetrad & genetic material is exchanged by crossing over
• Crossing over ensures greater genetic variation
Crossing OverCrossing Over
Meiosis I - Metaphase IMeiosis I - Metaphase I• Tetrads line up at the equator of the
cell
Meiosis I - Anaphase IMeiosis I - Anaphase I• The homologous chromosome
separate and move to the poles• Each pole receives a mixture of
maternal and paternal chromosomes
Meiosis I - Telophase IMeiosis I - Telophase I• Chromosomes decondense • The nuclear membrane may reform• Cytokinesis usually occurs
• Brief• Recondensation of
the chromosomes• Very similar to
conditions in prophase of mitosis
Meiosis II - Prophase IIMeiosis II - Prophase II
Metaphase II & Anaphase IIMetaphase II & Anaphase II
• Metaphase II- Chromosomes line up at the equator
• Anaphase II- the chromatids separate and are now called chromosomes
Telophase IITelophase II• There is one copy of
each homologous chromosome at each pole
• Nuclei form around chromatids
• Cytokinesis separates the cytoplasm
• At the end of meiosis, there are (typically) 4 haploid daughter cells
Mitosis vs. MeiosisMitosis vs. Meiosis• In mitosis a single division results in
two genetically identical daughter cells and there is no crossing over
• In meiosis, two sets of divisions occur resulting in four genetically different cells. A great deal of genetic diversity occurs caused by synapsis and independent assortment
Meiosis & Genetic VariationMeiosis & Genetic Variation• The events of meiosis & fertilization
are responsible for the variation in each new generation– Independent assortment– Crossing over– Random fertilization
Independent AssortmentIndependent Assortment• Contributes to genetic variability due
to random orientation of tetrads at metaphase plate
• 50-50 chance that a particular daughter cell of meiosis I will get the maternal chromosome, 50-50 chance that it will receive paternal chromosome
Crossing OverCrossing Over• Crossing over produces recombinant
chromosomes, which combine genes inherited from each parent
• Begins very early in prophase I - homologous chromosomes pair up gene by gene
• Homologous portions of two nonsister chromatids trade places.– Humans - occurs 2 – 3x/chromosome pair
Crossing OverCrossing Over• One sister chromatid may undergo
different patterns of crossing over than its “mate”
• Once these undergo independent assortment in meiosis II, variation of gametes increases even more!
FertilizationFertilization• An ovum is one of ~ 8 million
possible chromosome combinations
• Successful sperm represents one of 8 million different possibilities
• Resulting zygote - 1 in 70 trillion (223 x 223) possible combinations
• Crossing over adds even more variation
Taken together…Taken together…• All three mechanisms reshuffle the
various genes carried by individual members of a population
• Mutations, still to be discussed, are what ultimately create a population’s diversity of genes
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