2. nucleic acids, chromatin, gene expression, and mutations 2

what is replication

making of DNA from DNA

why is replication important

essential because whenever a cell divides, the two new daughter cells must contain the same genetic information, or DNA, as the parent cell

cell divides following _____

replication

DNA replication is _____

semiconservative

what is semiconservative

the two new DNA molecules each contain one parental strand and one new strand hat makesw

what makes DNA semiconservative

both the parenteral DNA strands are used as a template for the synthesis of a new DNA molecule

3 stages of DNA replication

• initiation

• elongation

• termination

what is initiation

• the DNA helix is opened and the DNA replication proteins are positioned

what is the process of initiation

• begins at the origin of replication

• from the origin of replication 2 replication forks are formed

• DNA synthesis proceeds bidirectionally

how many replication origin sites does bacterial DNA have

a single replication origin site

• slow

how many replication origin sites does eukaryotic DNA have

multiple

• DNA synthesis starts at all origins at the same time

• in human, 50,000 DNA replication origins

• fast

steps of DNA replication initiation

• helicase unwinds the DNA helix

• unwinding causes topological distortion of the DNA

• topoisomerase 1 prevents the topological distortion by breaking the DNA strand

• single-stranded DNA binding proteins prevents the two original strands from re-forming a double stranded molecule

what is elongation

synthesis of the new DNA strands

what does primase do in elongation

generates short RNA strand that binds to the singel-stranded DNA to initiate DNA synthesis

what does DNA polymerase do in elongation

• reads the nucleotide sequence of the parenteral strand in the 3’ → 5’ direction and synthesizes the new DNA strand 5’ → 3”

• adds nucleotide by forming phosphodiester bonds at the 3’ ends of the sugar moleculel

leading strand

continuous replication

lagging strand

discontinuous replication

what is being formed from lagging strand

short Okazaki fragments

what is ligase

and enzyme that links Okazaki fragments

coordination between the leading and lagging strands synthesis

• a looping of the template for the lagging strands places it in position for 5’ → 3’ polymerization

• looping enables DNA polymerase to synthesize both daughter strands simultaneously

what is termination

DNA synthesis is stopped

steps of DNA replication termination

• once DNA synthesis is completed, the RNA primer is removed by RNA hybridase

• DNA polymerase fills the gap where the RNA primer used to be

• DNA ligase seals the last two nucleotides together

how often do DNA polymerases make mistake

once every 10⁴-10⁶ deoxynucleotides

how does DNA polymerase fix mistakes

• proofread or 3’ → 5’ exonuclease activity

• DNA polymerase remove the mis-paired deoxynucleotides

DNA polymers inhibitors in cancer therapy

• Gemcitabine: a nucleoside analog

• incorporated into replicating DNA

• inhibit DNA pol and stall replication fork

• used to treat pancreatic, lung and bladder cancers

what is a telomere

telomere DNA sequence is added after replication to protect the ends of chromosomes in eukaryotes

what is a telomerase

a reverse transcriptase that uses and RNA molecule as a template

• add repetitive DNA sequence to six nucleotide after replication

• adds DNA to telomeres

cells that need to proliferate have _____

high telomerase activity

• white blood cells, stem cells, cancer cells

• most cells from healthy tissues have no or very little telomerase activity

what happens to telomere length

decreases with age in proliferating tissues

what does shortening of telomeres lead to

metabolic arrest, cell death or oncogenic transformation

telomere damage or deletion forms _____

reactive ends

why is telomere damage or deletion essential

essential for maintaining the integrity and stability of linear eukaryotic genomes

• loss of genome integrity, a characteristic feature of cancer

telomerase as a cancer target

telomerase is significantly over expressed in 80-95% of all malignant tumors

what is Epigallocatechin-3-gallate

a natural telomerase inhibitor present in green tea

• helps prevent cancer

what makes up chromatin

DNA and structural proteins

what is the fundamental unit of chromatin

nucleosome

chromatin appearance under microscope

in extended form, looks like beads on a string

nucleosome appearance and structure

• the bead on the the string

• has a core particle and a linker region

what forms do chromatin exist in

euchromatin and heterochromatin

characteristics of euchromatin

less condensed, lightly stained, and can be transcribe

characteristics of heterochromatin

highly condensed, darkly stained, and is typically not transcribed

what does chromatin form when condensed

chromosomes

how dos chromatin condense

• wraps around structural proteins in order to fit into the nucleus

• further condensed to form chromosomes

characteristics of mitochondrial DNA

• circular

• lacks DNA associated structural proteins

• lacks telomeres