BIO104 exam 3

Cell Cycle

The series of phases that a eukaryotic cell goes through to divide and replicate its DNA, including interphase and mitotic phase.

Cells are typically in interphase for 90% of life cycle; Mitosis 10%

Phases of cell cycle

Interphase: cell grows and functions normally

G1 phase - synthesize RNAs and proteins required for DNA synthesis

S phase - DNA replication + growth

G2 phase - cell makes proteins and RNAs in preparation for mitosis

M Phase: actual division process

Mitosis - cell division of nucleus where chromosomes have to be properly aligned

Cytokinesis - divison of cytoplasm, resulting in two daughter cells

Cell cycle checkpoints

ensure proper progression to avoid errors and mutations

G1 - checks DNA for problems/damage mutation, if conditons arent met it goes into G0 for resting

G2 - DNA copied currently, if wrong it dies (apoptosis)

M - ensures chromosomes are aligned in the middle

Phases of Mitosis

prophase, metaphase, anaphase, telophase, cytokinesis

Prophase

• nuclear envelope dissolves

• chromosomes duplicate and condense

Metaphase

• microtubules reach out to attach to sister chromatids

• sister chromatids line up at metaphase plate at center of cell

Anaphase

• sister chromatids separate from each other

• move to opposite poles/end of cell

Telophase

• sister chromatids finish moving to poles/end of cell

• nuclear envelopes start to form around them at each end

cytokinesis (final stage of cell division, not technically mitosis)

• cytoplasm splits, forming two identical daughter cells

• cleavage furrow forms in animal cells

Proto-oncogene (Ras in DNA)

normal function - code for growth factor proteins → block mitosis

mutated Ras (oncogene) - protein made will overstimulate the cell divison

growth factor

protein that binds the growth factor receptor on a cell to signal the cell to enter mitosis

normal function - growth factor binds receptor that activates

Ras protein - starts a signal pathway → cell makes proteins for cell division

Tumor supressor genes

normal function - code for a protein that stops mitosis to prevent tumor formation

TSG mutated - inhibitng GF binds R on cell → signaling pathway → mutated gene → protein that can not stop mitosis

cancer cells

do not follow the normal cell cycle stops/starts or cyclin activations (due to defect in the cell cycle control system)

• ignore the signal and continue to divide by piling up

Apoptosis

programmed cell death due to

• DNA mutations

• Outside stresses overwhelm the cell

  • cell dies

  • UV, beta, gamma radiation

  • toxins + other chemicals

structure of DNA

double helix: sugar (deoxyribose), phosphate group, nitrogenous base

what are the nitrogenous bases and what do they pair with?

Adenine (A) with Thymine (T)

Cytosine (C) with Guanine (G)

what is the DNA backbone made of?

phosphate (5’ end), deoxyribose sugar (3’ end)

why is DNA considered anti-parallel?

the strands run in opposite directions, necessary for replication and enzyme function

DNA replication steps

1. Hydrogen bonds between nucleotides break

2. Strands of DNA separate

3. Free nucleotides are attracted to exposed bases on the loose strands of DNA

4. Hydrogen bonds between nucleotides form

DNA replication enzymes

Helicase, Single-Stranded Binding proteins, Primase, Topoisomerase, DNA Polymerase I and III, Ligase

DNA helicase

unwinds DNA strands to create replication “bubble”, recognize the A-T rich sequences

Single-Stranded Binding Proteins

bind the unpaired DNA strands to stabilize them

Topoisomerase

keep the double helix stable just ahead of replication fork

DNA primase

adds RNA primers to start replication

DNA polymerase I

removes RNA nucleotides with DNA nucleotides

DNA polymerase III

creates new strand 5’ to 3’ direction

DNA ligase

joins DNA fragments together — seals gaps between Okazaki fragments

order of enzymes in DNA replication

Helicase → Topoisomerase → Single-Stranded Binding Proteins → Primase → DNA poly III → DNA poly I → Ligase

Leading Strand

requires only one RNA primer and is synthesized continuously in the 5’ to 3’ direction

Lagging strand

synthesized discontinuously in Okazaki fragments, DNA poly I replaces RNA primers with DNA nucleotides, DNA ligase joins the fragments to makes one long strand

replication bubble formation

begins at A-T rich sequences because they have less hydrogen bonds, helicase unwinds to create bubble and SSBPs stablize to keep them apart, the replication forks form at the ends for synthesis

Transcription (nucleus)

DNA → mRNA

Copy of gene’s sequence is made

Transcription steps - Initiation

transcription factors (proteins) attached to the promoter and help RNA poly II bind to the promoter (TATA box) on the gene = Transcription Initiation Complex (TIC)

RNA poly II unwinds/opens DNA double helix

Transcription steps - Elongation

RNA poly reads DNA nucleotides and matches the template using RNA nucleotides (synthesizes mRNA in the 5’ to 3’ direction) → RNA transcript continues till it reaches the terminator sequence

Transcription steps: Termination

RNA polymerase transcribes the polyadenylation signal sequence (AAUAAA) in the pre-mRNA

Accessory proteins cut RNA transcript off DNA → free floating pre-mRNA found in nucleus

Transcription: Termination - editing + splicing

create mature mRNA/ be protected when sent to cytoplasm

Introns and exons are involved in splicing

Introns

noncoding segments of the pre-mRNA that stay in the nucleus as space holders

Exons

code for protein + stay in the mature mRNA → leave the nucleus

types of RNA

mRNA, tRNA, rRNA

mRNA

messenger RNA (DNA→ ribosome)

tRNA

transfer RNA carries amino acids (aa’s) to ribosome to be linked

has an anticodon that pairs with mRNA codon

rRNA

ribosomal RNA combines accessory proteins to form ribosomes to link

Triplet Codon (on mRNA)

3 RNA nucleotides that code for a specific amino acid

Start codon

AUG (Methionine)

Stop codons

UAA, UAG, UGA (do not code for an amino acid)