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