Module 11: DNA Replication and Cell Division

why does cell division occur

cell growth, cell replacement, cell healing, cell reproduction

2 important requirements of cell division

the daughter cells must get all genetic material, parent needs to be big enough to turn into 2 new cells with enough cytoplasm

prokaryotic cell division

binary fission

eukaryotic cell division

mitosis and cytokinesis

steps of binary fission

proteins bind the circular genome to plasma membrane, replication begins at a specific location and proceed bidirectionally (the new DNA is also attached to the membrane), cell elongates do two times the size, constriction forms at the midpoint, new membrane and wall formed

genome in eukaryotes vs prokaryotes

large and linear and in nucleus; small and circular and in cytoplasm

cell cycle: 2 main stages

M phase (cell division) and interphase

what does M phase consist of

chromosome replication (mitosis) and cytoplasm division (cytokinesis)

what does interphase consist of

G1 phase, S phase, G2 phase, G0 phase

G1 phase

cell gets bigger and regulatory proteins are synthesized and activated in preparation for S phase

S phase

“synthesis” phase; DNA gets replicated

G2 phase

cell prepared for M phase

G0 phase

no active preparation for cell division, occurs in cells that don’t actively divide like liver cells

why is DNA replication described as semiconservative

each daughter strand only gets half its parent strand

helicase

unwinds the dna at the replication fork

single strand binding protein (SSBPs)

bind to the unwound DNA to prevent it from sticking back together

topoisomerase

works ahead of the replication fork to change the supercoiled state of dna and relive the stress

DNA polymerase

adds bases to the template DNA and can correct mistakes; requires deoxyribonucleotides dATP, dCTP, dGTP, dTTP

DNA replication direction

5’ to 3’ building

RNA primase

makes short pieces of RNA on the template strand; required so that DNA polymerase knows where to add bases

leading strand

DNA strand growing toward the replication fork and synthesized continuously as 1 big polymer

lagging strand

grows away from the replication stand and synthesized discontinuously

Okazaki fragments

short discontinuous pieces of dna formed on the lagging dna strand

how does dna synthesis on the lagging strand work

rna primer added by rna primase→ dna polymerase extends the primer → rna primer replaced by a different dna polymerase

DNA ligase

joins adjacent Okazaki fragments once the primers have been replaced

trombone model

leading and lagging strand synthesized at the same time by the looping of one of the DNA strands

DNA proofreading

most DNA polymerase can correct their own work; hydrogen bonds temporarily hold nucleotides in place before polymerase bonds it allowing opportunity for checking

what happens when dna polymerase finds a mistake

cleavage function is activated to remove the wrong nucleotide and replace it with the right one

how many replication forks in prokaryotic dna

2; move bidirectionally until they meet and fuse

how many replication forks in eukaryotic dna

multiple; creates bubbles of forks moving bidirectionally and they fuse when they meet via DNA ligase

starting point of replication; prokaryotic vs eukaryotic

1 origin in prokaryotic dna but multiple orgins in eukaryotic dna

what is the issue when replication reaches the end of the dna in eukaryotes

final primer on the lagging stand is added 100 nucleotides from the 3’ end, leaving a section of DNA unreplicated; dna shortens every time it is replicated

telomeres

ends of linear chromosomes

telomere sequence

5’ TTAGGG 3’ repeated 1.5k-3k times on each telomere

telomerase

ribonucleoprotein (protein-rna complex), replace missing nucleotides on telomeres to address shortening, only in some cell types

which cells have active telemerase

stems cells and germ cells (sex cells)

which cells have inactive telemerase

adult somatic cells

hayflick limit

mitotic division only happens 50 times in somatic cells before telomeres become too short and division stops

characteristics of telomeraase

carries its own primer (template rna) and has reverse transcriptase activity (rna →dna)

how does telomerase work

adds nucleotides to 3’OH end of lagging strand to prevent shortening; uses its own primer complementary to telomeric repeats. then primer can be added regularly and dna polymerase can continue building

what happens with dna overhang on telomeres

forms loops on the ends of the chromosome for degradation protection

haploid

cell with one copy of each chromosome

diploid

cell with 2 copies of each chromosome

sister chromatids

2 identical copies of a chromosome; make the x shape

why is it hard to see chromosomes during interphase

not condensed

karyokinesis

nuclear division, same as mitosis

mitosis: prophase

• chromosomes become visible

• centrosomes duplicate and go to poles

• mitotic spindle extend from centrosomes

mitotic spindle

made of microtubules that pull centrosomes apart

centrosomes

microtubule-organizing centers in animal cells, don’t exist in plant cellls

prophase in animals vs plants

both have microtubule-based mitotic spindle, but only animal cells have centrosomes

mitosis: prometaphase

• nuclear membrane disappears

• mitotic spindle grow and shrink to explore → attach to chromatid’s kinetochore

kinetochores

protein complexes associated with a chromosome’s centromere; 1 on each side

mitosis: metaphase

spindle lengthen and shorten to align chromosomes at the metaphase plate

metaphase plate

center of the cell; equidistant from both spindle poles

mitosis: anaphase

microtubules shorten to split the centromere → one chromatid from each pair at each pole of the cell

mitosis: telophase

• complete set of chromosomes at each pole

• cytosolic changes in preparation for cell division

• mitotic spindle disintegrate

• 2 new nuclear envelopes reform → chromosomes decondense

animal cells: cytokinesis

contractile ring contracts; pinches the cytoplasm of the cell to form 2 daughter cells

contractile ring

ring of actin filaments on inner part of the cell membrane; forms at equator of cell perpendicular to the axis of the spindle

cell plate

fused vesicles in the middle of a dividing plant cell; forms a cell wall during late anaphase and telophase

phragmoplast

forms in plant cells; made up of overlapping microtubules that guides vesicles containing cell wall to the middle of the cell in telophase

plant cells: cytokinesis

cell plate fuses with original cell wall at the perimeter of the cell

what are the 2 critical points if the cell cycle

initiation of dna replication (g1→s transition) and initiation of mitosis(g2 → m transition); require regulation

cyclins

regulatory protein subunits of specific protein kinases

CDKs

cyclin-dependent kinase; they phosphorylate proteins that promote cell division. always present in the cell and need cyclin to activate

what is the purpose of the different kinds of cyclins and CDKs

act at different steps of the cell cycle

G1 phase regulation

levels of cyclin D and E rise → activates CDKs in preparation for the S phase

example of g1 phase regulation

cyclin d and cdks activate transcription factors that lead to expression of dna polymerase

s phase regulation

cyclin A levels increase → cdks activated → dna synthesis initiation

G2 phase regulation

cyclin b level rises → CDKs activated → mitosis prep like nuclear envelope breakdown and mitotic spindle formation

3 major checkpoints in the cell cycle

dna damage checkpoint, dna replication checkpoint, spindle assembly checkpoint

dna damage checkpoint

check for damaged dna before entering s phase

what happens if the dna is found to be damaged by radiation during the checkpoint

protein kinase activated → phosphorylation of p53 protein → p53 levels increase in nucleus → activates transcription of inhibitors to block G1/S cyclin-cdk complex → repair time

what is known as the guardian of the genome

p53

dna replication checkpoint

end of G2; checks for unreplicated DNA

spindle assembly checkpoint

checks that all chromosomes are attached to spindle before cell progresses with mitosis in proanaphase

why does the dna need to repair itself quickly when damage is detected

phosphorylated p53 stimulates Bax protein and suppresses Bcl-2 protien transcription → Bax making dimers with itself rather than with Bcl-2

what gene codes for bax protein

bax gene

what gene codes for bcl-2 protein

bcl-2 gene

bax/bcl-2 in healthy cells

balanced concentrations; they form bax/bacl-2 dimers

bax/bcl-2 in unhealthy cells

not enough bcl-2 leading to bax/bax dimers forming and apoptosis

apoptosis

controlled and orderly disintegration of the cell when Bax/Bax dimers increase

why is apoptosis important in embryos

allows remodeling of tissues (eg hand looks like a paddle until cells are selectively killed)

why is apoptosis important in adults

tissue size maintenance (balances cell multiplication), elimination of cells no longer needed (T lymphocytes), elimination of damaged cells

cancer

uncontrolled cell division

oncogenes

cancer-causing genes; first discovered in viruses. can arise from mutated proto-oncogenes (normal genes that promote cell division)

tumor suppressor gene

inhibits cell division, for example, p53

the key to cell division is…

proto-oncogenes that promote cell division and tumor suppressor genes that inhibit division

key features of cancer cells

• divide on their own without growth signals

• resist inhibition and cell death signals

• metastasis

• angiogenesis

metasasis

ability to invade tissues

angiogenesis

new blood vessel formation

benign cancer arises from

oncogene activation

malignant cancer arises from

inactivation of second tumor suppressor gene

metastatic cancer

inactivation of the third tumor suppressor gene