Week 5 - DNA Replication, Transcription and Translation Flashcards
What are the subunits of DNA and RNA?
nucleotides
What does each nucleotide contain?
a five carbon sugar, a nitrogenous, base, a phophate group
What is five carbon sugar in DNA?
deoxyribose
What is a nucleoside?
base + sugar (no phosphate group)
eg adenosine, guanosine
How are sugars in nucleic acids linked to one another?
by phosphodiester bridges
What is a nucleosome?
a particle made of histone proteins with approximately two turns of DNA wrapped around them
How many hydorgen bonds hold together G and C?
3, A and T linked by 2
What is needed for DNA replication?
single stranded template
DNA polymerase enzyme
deoxynucleoside triphosphates (dATP, dCTP ect)
a primer
What is a primer?
a short piece of nucleic acid base-paired to the template, acts as a start point, a 3’ -OH group
What enzyme catalyses the formation of the primer?
primase - an RNA polymerase that synthesises a short stretch of RNA complementary to the DNA strand
What enzyme partially unwinds the DNA replication fork?
DNA helicase
In what direction does DNA synthesis occur?
5’ to 3’
Which strand is synthesised continously?
the leading strand
How is the lagging strand synthesised?
as short fragments called Okazaki fragments
RNA primer is degraded ands replaced with upstream Okazaki fragment acting as primer
What enzyme extends the ends of chromosones?
In what cells does this occur?
telomerase
germ-line cells and to a lesser degree stem cells
telomerase is also found in cancer cells
What is the end replication problem?
On the lagging strand there is an incomplete 5’ end after removal of the RNA primer which cannot be filled because there is no upstream Okazaki fragment
chromosones shorten each time they replicate
What is an intron?
a non-coding portion of DNA
DNA sequences that are present in the gene but not in the corresponding RNA
What is an exon?
a coding portion of DNA
What is a promotor?
a DNA sequence that signals the start of a gene
a binding site for RNA polymerase
What is a codon?
a triplet of nucleotides representing an amino acid
eg UUU - Phe, AGU - Ser
What does the term degenerate mean in reference to the genetic code?
same amino acid may be represented by more than one codon
eg Asn - AAU and AAC
What is the initiation codon?
AUG
What are the three termination codons?
UGA, UAG, UAA
What is translation?
synthesis of a protein from mRNA
second step in gene expression
To what part of the tRNA molecule does mRNA bind?
the anticodon
What is an aminoacyl tRNA?
a tRNA attached to their amino acid
an amino acid is ‘activated’ by attaching to tRNA
How many different types of aminoacyl-tRNA synthetases are there?
20
they are the enymes that link tRNAs to their aminoacids and there most be 20 different types for each type of amino acid
ATP is required for the reaction
What is the overall size of the human ribosome?
80S
large subunit - 60S
small subunit 40S
DO NOT add them together to make 100
What size are mitochondrial ribosomes?
55S
What are the three tRNA binding sites on the ribosome?
P (peptidyl) site, the A (aminoacyl) site and the E (exit site)
What does the translation cycle involve?
binding of aminoacyl-tRNA (requires GTP)
the peptidyl transferase reaction (peptide bond synthesis)
translocation (movement of the ribosome one codon along the mRNA - needs GTP)
What site does the first aminoacyl-tRNA (Met) bind to on the ribosome?
P site
What is the mechanism of action of the antibiotics ethryomycin and clarithromycin?
inhibits bacterial protein synthesis by inhibiting the peptidyl transferase reaction and translocation
What is purpose of the peptidyl-transferase reaction?
to form peptide bonds between adjacent amino acids during translocation
What enzyme carries out transcription?
RNA polymerase
What is the process of removing introns called?
splicing
What DNA bases are purines?
adenine and guanine
2 ring structure
