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molecular structure of DNA
double helix
phosphate group
deoxyribose (sugar)
nitrogenous base sticking out to the side
DNA backbone structure
covalent bond between a phosphate group and deoxyribose
purines
adenine (A)
guanine (G)
double ring structure
pyrimidines
thymine (T)
cytosine (C)
single ring structure
rule of base pairing
only C and G can form a triple hydrogen bond with each other
only A and T can form a double hydrogen bond with each other
amount of cytosine = thymine, amount of guanine = adenine
double ringed base has to be attached to a single ringed base
antiparallelism
strands run in opposite directions
5’ tail, 3’ tail
problem: can only easily add onto the 5’ end
when does DNA replication occur?
S phase
DNA replication
two original strands are used as a template for new DNA based on the rules of base pairing
results in 2 identical molecules of DNA
conservative model
the two parental strands reassociate after acting as templates for new strands, thus restoring the parental double helix (comes back together)
semiconservative model
the two strands of the parental molecule separate, and each functions as a template for synthesis of a new, complementary strand
actual model
½ original, ½ new strand
steps of DNA replication
initiation: unwinding of DNA helix at the origin of replication
elongation: new strands are synthesized by DNA polymerase from the template strands
leading strand synthesis: continuous synthesis on the strand towards the replication fork
lagging strand synthesis: discontinuous synthesis, producing Okazaki fragments away from the fork
termination: complete synthesis and separation of the newly formed DNA strands.
DNA helicase
enzyme that breaks the hdrogen bonds between nucleotides and splits “unzips” the helix into 2 parts
lagging strand
replicates discontinuously forming short Okazaki fragments
can’t add onto 5’ end
runs 5’ to 3’ away from the fork
leading strand
replicates continuously
can easily add onto 5’ end
runs 5’ to 3’ towards the fork
smooth process because helicase moving in same direction (reading 5’ - 3)
primase
an enzyme that synthesizes a short RNA primer to provide a starting point for DNA synthesis during replication
DNA polymer I (DNA pol I)
enzyme responsible for removing RNA primers and filling in the gaps with DNA nucleotides, proofreading and correcting
DNA polymer III (DNA pol III)
enzyme responsible for synthesizing new DNA strands during replication, working in conjunction with the leading and lagging strands
topoisomerase
relieves the strain from winding up the DNA, makes it more flexible and easier to work with
single-strand binding protein
prevents the original strands from re-bonding
proofreading in prokaryotes
no proofreading mechanism allows for more variation due to higher mutation rates
mismatch repair
a cellular mechanism that detects and rectifies errors made during DNA replication, maintains accuracy of genetic information
nucleotide excision repair
nuclease cuts out the damaged DNA strand at two points, damaged section is removed
DNA polymerase fills in the section with repair synthesis
telomere
tiny caps on the end of DNA strands
repeated DNA base sequence that is non-coding
protect the coding DNA as every time the DNA is replicated it gets a little bit shorter
telomerase
enzymes that prevents DNA from becoming shorter
protect telomere
ligase
enzyme that joins DNA fragments by forming covalent bonds, sealing nicks and linking Okazaki fragments
protein synthesis - transcription
a segment of DNA is taken and a complementary mRNA is formed
protein synthesis - translation
codons are translated into a chain of amino acids (polypeptide)
protein synthesis - ribosome
made up of a large and small subunit
UTR helps mRNA bind to the small subunit
P site (polypeptide) contains tRNA with developing polypeptide
A site (arrival) is where the next tRNA will arrive and enter the ribosome
E site (exit) is where tRNA sits right before it leaves during translation
building a polypeptide - initiation
mRNA binds to small subunit
start codon (AUG) signals amino acids and forms a hydrogen bond with codon
utilizing energy from GTP, large ribosomal subunit sandwiches mRNA and small subunit
E site empty, A site empty, P site with codon
building a phosphate - elongation
assembly line of workers bring amino acids over one at a time
translation has occurred a few times
P site growing polypeptide chain
tRNA carrying a new amino acid enters A site and bonds polypeptide chain to the new amino acid
building a polypeptide - termination
tRNA that was in the P site is now in the A site, tRNA that was in the E site exits
A site now empty and ready to accept another tRNA
termination occurs from stop codons (UGA, UAA, UAG) that signal ribosomes to stop translating and for the polypeptide chain to be released
polyribosome
different ribosomes translate the same mRNA molecule to produce polypeptides at the same time, allows cells to produce a lot of a protein at once
ribosomes in the cytoplasm
proteins will be used in the cell
all ribosomes start in cytoplasm
ribosomes in the rough ER
proteins will be secreted outside of the cell, be used in the cell membrane, or become a lysosome or peroxisome
signal peptide signals the cell that the protein is needed outside of the cell, and should move over to the ER
translocation complex opens up and as translation occurs the polypeptide starts to enter the lumen of the ER as its being made
silent mutation
one base changes, but the amino acid coded for stays the same
no change in amino acid sequence → no change In protein
missense mutation
changes a single amino acid
nonsense mutation
amino acid coded for changes to premature stop codons instead of another amino acid
causes polypeptides being much shorter than they should be
frameshift mutation
base is inserted and all bases shift to the right
can also occur when a base is deleted and all bases are shifted in opposite directions, as well as every amino acid that follows that interaction
