Genetics Chapter 2: Cell Divison and Punnett

chromosomes

carry genetic information as a string of nucleotides, which are contained in genes; must be duplicated for cell division to occur

mitosis

used for growth, repair, and maintenance of diploid cells

meiosis

used for haploid gamete production from diploid precursors

spindle fibers

move eukaryotic chromosomes into position; attach to the centromere on either side of sister chromatids; promotes bi-orientation

bi-orientation

a stage where chromosomes are pushed to the cell center form each side; detection of tensional equilibrium (bipolarity) ensures chromosomes are in correct position for cell division to occur

disjunction

separation of sister chromatids when chromosomes are not lined up properly, which is a good thing

nondisjunction

when spindle fibers fail to attach properly to chromosomes so the cell divides with unequal numbers of chromosomes

Interphase

G1: cell grows, organelles produced, division enzymes are made

S: centromere duplicates first, then DNA, then the centrosomes

G2: more growth, more energy generated, more enzymes made

Interphase checkpoints

G1: ensures cell is healthy and ready for division; cyclin proteins and kinases must be readily available to mediate check points

S: ensures DNA replication forks are stable

G2: ensures DNA replicated properly during S phase

M: ensures spindles are attached properly so that all the chromosomes are aligned at cell center to allow disjunction to occur

prophase

chromosomes condense, nuclear membrane and nucleoli break apart and disappear, spindle fibers begin to form from microtubule organizing centers

prometaphase

spindles attach to kinetochores of centromere on both sides of the sister chromatids (centomere integrity is maintained)

metaphase

tethered chromosomes are pushed to cell center

anaphase

sister chromatids are separated into daughter chromosomes as they are pulled back to opposite sides of the cell

telophase

two nuclei form around each set of daughter chromosomes within a single cell, which then leads to cell division

cytokinesis

cell divides in half into two identical new cells; plant cells use a cell plate, animal cells use a cleavage furrow

chiasmata

maintain tetrad formation in prophase I-metaphase I

condensin

helps eukaryotic chromosomes coil up

cohesin

holds sister chromatids together; begins to be digested from the telomeric ends inwards from G2 on; last bit digested at end of metaphase

shugoshin

interacts with cohesin; helps orientate kinetochores to face spindles

Meiosis I

the tetrads synapse, cross-over in Holliday junctions, and separate: one diploid cell is reduced to two haploid cells

Prophase 1 - homologs condense, synapse, cross-over, spindles attach

Metaphase 1 - tetrads pushed to cell center by spindle fibers

Anaphase 1 - tetrads separated into recombined sister chromatids

Telophase 1 - two haploid nuclei form inside one cell

Interkinesis

two haploid cells form after division

Meiosis II

two haploid cells give rise to four haploid cells

Prophase 2 - haploid pairs of sister chromatids condense, spindles attach

Metaphase 2 - sister chromatids pushed to cell center

Anaphase 2 - sister chromatids separated into daughter chromosomes

Telophase 2 - two haploid nuclei reform in each cell

cytokinesis

four haploid gametes will appear for human males; in females, however, three of the cells degenerate leaving behind only one giant oocyte

crossing-over

occurs in Meiosis I, allowing the enzyme SPO11 to make double-stranded DNA breaks resulting in some allelic changes between homologs*

separase

digests cohesin allowing cell into Anaphase (I, II)

alleles

variations of a gene

phenotype

a trait

genotype

the alleles that produce a trait

Mendelian inheritcance

indepdendent assortment of alleles

haplosufficient

in a heterozygote state, only one of the two alleles is necessary to produce enough gene product

haploinsufficient

any condition when a single allele is not sufficient to produce enough functional protein

pseudofunctionalization

results in gene activation

neofunctionalization

produces a novel gene function

subfuncitonalization

when genes share a function

tester

organism with all recessive alleles

test crossing

ascertains if an organism is displaying a particular phenotype

4:0

phenotypic or genotypic ratio; F₁ outcome from crossing pure-bred homozygous parents

3:1

phenotypic ratio; F₂ outcome if crossing monohybrid F₁ heterozygotes

1:2:1

genotypic ratio; F₂ outcome if crossing monohybrid F₁ heterozygotes

1:1

phenotypic or genotypic ratio; F₂ tester outcome for monohypbrid heterozygote X tester

digenic

ratio exhibiting 4 phenotypes

9:3:3:1

phenotypic ratio; F₂ outcome if crossing dihybrid F₁ heterozygotes

1:2:1:2:4:2:1:2:1

genotypic ratio; F₂ outcome if crossing dihybrid F₁ heterozygotes

1:1:1:1

phenotypic or genotypic ratio; F₂ outcome if testing a dihybrid heterozygote