Lecture 17 Transcription translation Flashcards
What are major phenotypic differences the result of?
Differences in specific proteins
What are the two major steps that express a gene?
Transcription and translation
What is transcription?
Information of a DNA sequence is copied into the corresponding information in an RNA sequence
What does translation do?
Convert RNA sequences into an amino acid sequence of a polypeptide
What is the purpose of RNA?
To be an intermediary between DNA and proteins
How does ribonucleic acid differ from DNA?
RNA is only one polynucleotide strand
RNA has ribose, rather than deoxyribose
RNA has uracil instead of thymine
How is uracil different to thymine?
It lacks a methyl group
What is the central dogma?
DNA codes for the production of RNA
RNA codes for the production of protein
Protein does not code for RNA or DNA
What theory did Crick propose to explain how information gets from the nucleus to the cytoplasm?
Messenger hypothesis
What is the messenger hypothesis?
RNA forms as a complementary copy of DNA of a particular gene
This messenger RNA (mRNA) travels from the nucleus o the cytoplasm
What did Crick propose to explain how a DNA sequence gets transformed into the specific amino acid sequence of a polypeptide?
The adapter hypothesis
What is the adapter hypothesis?
There must be an adapter molecule that can bind a specific amino acid sequence and recognize a sequence of nucleotides
What were found to be adapter molecules?
Transfer RNA (tRNA)
Why is it said that tRNA translates DNA?
Because they recognize the genetic message of mRNA and simultaneously carry specific amino acids
What happens during translation?
tRNA carrying bound amino acids line up on the mRNA sequence so the amino acids are in the correct sequence
How has the adapter hypothesis been confirmed?
Actual observation
What exceptions are there to the central dogma?
RNA viruses
Give some examples of viruses that have RNA genetic material.
Tobacco mosaic virus, influenza virus, poliovirus
How do RNA viruses replicate?
They make a complementary RNA strand to their genome which is used to make copies of the viral genome by transcription
What is synthesis of DNA from RNA called?
Reverse transcription
Name a retrovirus.
HIV
What type of genetic information does HIV have?
RNA
How does HIV replicate after infecting a host cell?
By making a DNA copy of their genome and using it to make RNA to use as a template and as mRNA to produce viral proteins
What enzyme is used in reverse transcription to synthesize DNA from RNA?
Reverse transcriptase
What components are needed for transcription?
- DNA template
- Ribonucleoside triphosphates (ATP, GTP, CTP, UTP) (substrates)
- RNA polymerase enzyme
What name is given to strand that is transcribed during transcription?
The template strand
What is the name given to the complementary strand that is not transcribed?
Non-template strand
What is synthesized during transcription?
mRNA, tRNA ribosomal RNA (rRNA)
What does RNA polymerase do?
Catalyses the addition of nucleotides in the 5’ to 3’ direction, processive
How many RNA polymerases are there in bacteria?
one
How many RNA polymerases are there in eukaryotes?
Three
What is the common structure of RNA polymerases?
Crab claw structure
What is the first step in RNA polymerase catalysis?
The enzyme recognizes certain bases within the DNA double helix and binds to them
What does RNA polymerase do after binding to certain bases on the DNA?
The ‘pincers’ close, keeping DNA in a double strand form called a closed complex
What happens once RNA polymerase has bound to DNA with a closed complex?
A conformational change in the RNA polymerase occurs, denaturing a short (10 base pairs) stretch of DNA and forming an open complex.
What happens after an open complex has formed?
Unpaired bases within DNA are available to pair with ribonucleotides, RNA synthesis begins
In what direction does the RNA polymerase read the DNA template strand?
from 3’ to 5’
How is RNA polymerases different from DNA polymerases during initiation?
Does not require a primer.
What three processes can transcription be divided into?
- Initiation
- Elongation
- Termination
What step begins transcription and what is required?
Initiation
A promoter
What is a promoter?
A sequence of DNA to which RNA polymerases bind very tightly
What do promoter sequences tell RNA polymerases?
- Where to start transcription
- Which strand of DNA to transcribe
- The direction to take from the start
How many promoters are there?
At least one for each gene
What part of the promoter is where transcription begins?
The initiation site
When does elongation start?
Once the RNA polymerase is bound to the promoter
How are DNA polymerases different to RNA polymerases during elongation?
RNA polymerases do not proof-read their work
Why are RNA polymerase errors not as serious as DNA polymerase errors?
Many copies of RNA are made, and they have a relatively short life span
(Errors are not as harmful)
What is termination controlled by?
Specific sequences of bases
How is termination of transcription different in prokaryotes compared to eukaryotes?
Prokaryotes- translation can start before transcription is finished
Eukaryotes- spatial separation, pre-mRNA
What is a codon?
Three nucleotides in a messenger RNA that direct the placement of a particular amino acid into a polypeptide chain
What is a start codon?
The initiation signal for translation
What is a stop codon?
Termination signal for translation
What happens when translation machinery reaches one of the stop codons?
Translation stops, the polypeptide is released from the translation complex.
What is the codon for a start codon?
AUG
What are the three stop codons?
UAA
UAG
UGA
What is the name given to genetic code to describe how there is more than one codon for each amino acid?
Redundant
What two amino acids are represented by just one codon each?
Tryptophan
Methionine
What other quality describes the genetic code?
It is not ambigous
What does it mean that genetic code is not ambigous?
A codon can only code for one amino acid
What does it mean that the genetic code is (nearly) universal?
In almost every species, codons that specify amino acids are the same
What are does the universal nature of the genetic code imply?
That the code is ancient and has been maintained throughout the evolution of living organisms
Where are some exceptions to the universal nature of genetic code be seen in plants?
Within mitochondria of chloroplasts the code differs
Other than plants, where can exceptions to the universal nature of the genetic code be seen?
In one group of protists, UAA and UAG code for glutamine rather than functioning as stop codons
What 2 key events during translation must take place to ensure that a protein made is the one specified by mRNA?
- tRNA must read mRNA codons correctly
- tRNA must deliver the amino acids that correspond to the mRNA codons it has read
What happens after tRNA’s have delivered the appropriate amino acids?
The ribosome catalyzes the formation of peptide bonds between amino acids
What are the three functions of the tRNA molecule?
- Carries (is charged with) an amino acid
- Associates with the mRNA molecules
- Interacts with ribosomes
How many nucleotides does a tRNA molecule have?
75-80
How is the conformation of a tRNA molecule maintained?
Complementary base pairing (hydrogen bonding) within its own sequence
What is at the 3’ end of every tRNA molecule?
Its amino acid attachment site
How do amino acids attach to the 3’ end of the tRNA molecule?
By binding covalently