Week 6: Cell Cycle and Mitosis

why do cells divide?

growth (grow the body), repair (refill damage), reproduction

cell division in prokaryotes

binary fission

plastid

very tightly folded circular chromosome that stores the DNA of prokaryotes

3 steps to prokaryotic division

1. chromosome attaches to plasma membrane at ori site

2. chromosome replicates, at which point there are two ori sites, one on either side of the cell

3. when chromosomes are complete, cell membrane divides by fission resulting in two daughter cells

difference with eukaryotic replication

cell cycle is regulated

complete set of DNA for an organism

genome

gene

segment of DNA that encodes for synthesis of one specific protein

chromosome

long, continuous strand of DNA containing hundreds to thousands of genes, that is tightly wound around histone proteins to remain compact

chromosomes in humans

somatic cells: 46 paired chromosomes

gamete cells: 23 unpaired chromosomes

how many genes do humans have

estimated 21,000

diploid cell

contains two copies of each chromosome, one from female parent and one from male parent (homologues)

haploid cells

contains one copy of each chromosome

slight differences in genes

alleles: codes for proteins with slight differences

genetic locus (loci)

location of a particular gene on a chromosome

duplicated chromosome

a larger chromosome consisting of two copies of an original chromosome (sister chromatids), joined at the center by centromere, and held together by cohesin proteins throughout length

centromeres

repetitive sequences of DNA bound by centromeric proteins

tetelomeres

“arms of sister chromatids” bridged by cohesin proteins

chromatin structure

segments of DNA wind around octomer histone proteins, kept in place by polar interactions (DNA negative - histone positive) and H1 clamps to form a more compacted string, which then folds into high order structures

nucleosome

histone with DNA wrapped around it, around 10nm in diameter

linker DNA

DNA in between nucleosomes in chromatin chain

karyotype

the complete set of unique chromosomes of an organism (in humans differs by one chromosome XX vs XY)

4 phases of the cell cycle

M: mitosis, nucleus divides and cell divides

G1: growth 1, cells perform regular functions

S: DNA synthesis, DNA is replicated

G2: growth 2, organelles are replicated

3 checkpoints in cell cycle

G1 checkpoint: checks internal and external conditions to approve DNA replication, checks for DNA damage

G2 checkpoint: checks that all organelles have been duplicated

M checkpoint: checks that all chromosomes are properly aligned

cytokinesis

division of cell membrane to separate two daughter cells

cyclin dependent kinase (CDK)

with the binding of cyclin to an active site, a protein is phosphorylated with a phosphate from ATP, activating the protein

cyclin

a protein made at certain points in the cell cycle, that binds to CDK to change its shape and expose an active site

MPF (maturation promoting factor)

CDK and cyclin combination that promotes cell passage into M-phase at the G2 checkpoint

5 stages of mitosis

prophase, prometaphase, metaphase, anaphase, telophase

interphase

pre-mitosis state: chromosomes are duplicated but not condensed, nucleolus is present, two centromeres formed

prophase

chromatin condenses X shapes composed of pairs of sister chromatids joined by centromeres, nucleolus disappears, mitotic spindle begins to form as centrosomes move to opposite sides

prometaphase

the nuclear envelope breaks down, exposing chromosomes to cytoplasm, microtubules extend from each centrosome

kinetochore

protein structure on outside of centromere that receives and attaches microtubules

metaphase

chromosomes arrive at metaphase plane (middle of cell), line up, and attach microtubules to kinetochores

mitotic spindle

a structure formed by polar microtubules that separates chromosomes during mitosis

movement of chromosomes in mitosis

motor proteins use ATP to move chromosomes detach from kinetochores and move along the microtubules to each side of cell

kinesin

motor protein that moves towards the positive end (kinetochores)

dyenin

motor protein that moves toward negative end (centrosomes)

anaphase

cohesin proteins degrade, allowing sister chromatids to part and microtubules shrink and reel daughter chromatids (now each chromosomes) toward centromeres, polar microtubules extend, pushing cell sides apart

telophase

two separate nuclear envelopes are constructed from parent cell, nucleoli reappear, chromosomes decondense, microtubules depolymerize

cytokinesis in animal cells

myosin motor protein pinches microfilaments cleaving cells

cytokinesis in plant cells

vesicles carrying cell wall materials join at the center of the cell, forming a middle wall which is split to form two daughter cells