Molecular Biology & Genetics 4- Transcription & Translation Flashcards
What does the Central Dogma of Molecular Biology show?
How information flows in the cell
What is the flow of information in the original model of the central dogma of molecular biology?
DNA (information) >transcription> RNA (messenger) >translation> Protein (worker)
What is the flow of information in the more recent and complex model of the central dogma of molecular biology?
DNA (information) +replication >transcription and reverse transcription> RNA (messenger) + replication >translation> Protein (worker)
What is gene expression?
The process by which information from a gene is used in the synthesis of a functional gene product
What are functional gene products?
Proteins and non-coding RNA
What is non-coding RNA?
An RNA molecule which won’t give rise to a protein, it instead has some other function
What is a gene?
A defined region (sequence) of DNA that produces a type of RNA molecule that has some function
What are the 3 possible functions of gene sequences?
Responsible for the regulation of synthesis of RNA, producing RNA and being responsible for the further processing of RNA
What is transcription?
DNA-dependent RNA synthesis where a double stranded DNA molecule is used as instructions to make a single stranded RNA molecule
What catalyses transcription?
RNA Polymerase
What does RNA Polymerase do?
Synthesises mRNA by catalysing the formation of phosphodiester bonds between ribonucleotides. It also selects the correct nucleotides to incorporate into mRNA based on the sequence of DNA which is being transcribed
What are the two DNA strands called in relation to transcription?
Coding strand (5’>3’) and template/non-coding strand (3’>5’)
What is the coding strand?
It contains the information which the cell needs
What is the non-coding strand?
The DNA strand which is complementary to the coding strand and contains information which isn’t needed by the cell
What strand of DNA is used as the template?
The non-coding strand
Why is the non-coding strand used as the template?
So that the RNA strand produced (RNA transcript) is complementary and contains the information found on the coding strand
What are the three steps of transcription?
Initiation, Elongation and Termination
In basic terms what is initiation?
The starting of transcription
In basic terms what is elongation?
The addition of nucleotides
In basic terms what is termination?
When transcription stops
What is found before the coding region of a gene?
The promotor region
What is within the promoter region?
An AT rich region called the TATA box
What do transcription factors do?
Binds to the TATA box and other regions of the promoter region
What happens once the transcription factors are bound?
The RNA Polymerase II binds to the area so that together RNA Polymerase II and the transcription factors can form the transcriptional initiation complex
What happens after the transcriptional initiation complex is formed?
The two DNA strands separate (10-20 nucleotides at a time) and RNA Polymerase II starts mRNA synthesis without the need of a primer - the RNA transcription has a 3’ OH group
What happens during elongation?
RNA Polymerase II uses the template strand, which runs in the 3’ > 5’ direction, as a template and inserts complementary RNA nucleotides in the 5’ > 3’ direction
What is the coding sequence in eukaryotic genes?
Portion of a genes DNA that is translated into a protein
What is the promoter in eukaryotic genes?
A DNA segment recognised by RNA polymerase to initiate transcription
What are UTR’s?
Untranslated regions of a eukaryotic gene which are transcribed but not translated
What do UTR’s contain?
Regulatory elements (sequences) that influence on gene expression at the transcriptional/translational level
What does the 5’ UTR do?
Facilitate the addition of the 5’ G cap (G=guanine)
What does the 3’ UTR do?
Facilitate the addition go the 3’ Poly-A tail
What is the function of the 5’ G cap?
Preventing mRNA degradation, promoting intron excision and providing a binding site for small ribosomal subunits
What is the Poly-A tail?
A long stretch of Adenine nucleotides
What is the function the Poly-A tail?
Preventing RNA degradation and facilitating the export of the mRNA from the nucleus into the cytoplasm through nuclear pores
What is the sequence of a gene in DNA?
Promoter which contains a TATA box, 5’ UTR, Coding sequence, 3’ UTR
What is the sequence of a gene in mRNA?
5’ G cap, 5’ UTR, coding sequence, 3’ UTR, Poly-A tail
What is the sequence of a gene in a protein?
Coding sequence only
The coding sequence in eukaryotes isn’t continuous, …
it is broken into regions with introns between
What are introns?
Non-coding begins which will be transcribed to form pre-mRNA (precursor mRNA) before being removed to form mRNA (mature RNA)
How are introns removed?
By splicing
In mRNA the coding region is…
continuous and therefore a protein can now be made
Where are non-coding regions found in eukaryotic genes?
Upstream and downstream of the coding sequence as well as within the coding sequence
What are non-coding DNA elements involved in?
Regulating gene expression
What would happen if changes “mutations” occurred in the non-coding gene sequence?
They may disrupt normal gene expression
Where does transcription occur in prokaryotes?
The cytoplasm
Where does transcription occur in eukaryotes?
The nucleus
After discovering DNA, what was the question scientists had?
How does the cell get from one macromolecule (nucleic acid) to another (protein) when they are structurally and chemically very different from each other?
How many amino acids and bases are there?
20 amino acids and 4 bases
More than enough information is given by…
triplets so there will be some redundancy
How much information is given by triplets?
There is 64 possible combinations
What is the genetic code based of?
Triplet codon hypothesis
What is a codon?
Three nucleotides that together specify a particular amino acid
How many codons specify an amino acid?
61 of the 64
How many codons do most amino acids have?
More than one although a few just have one
What do the. codons which don’t specify an amino acid do?
Cause translation to stop
What codon specifies start?
AUG
What is the starting amino acid?
Methionine- it will always be the first amino acid in a sequence
How is the genetic code read?
The amino acid is carried to the template by an adaptor molecule and that adaptor is the part which usually fits onto the RNA
What is the adaptor molecule?
tRNA
What is the structure of tRNA?
It is a single strand of 70-80 RNA nucleotides. There is at least one tRNA for each amino acid and it has a 3 dimensional structure which fits ribosomes like a glove
What does each tRNA have?
A region which can bind an amino acid and a region which can interact with mRNA
How many loops are in a tRNA molecule?
3 which are all important
Where is the amino acid attachment site on tRNA?
The 3’ end
Where is the part which interacts with mRNA on tRNA?
At the tip of the bottom loop- 3 nucleotides (anticodon) bind to interact with the mRNA
How does tRNA become charged?
An enzyme recognises both a specific amino acid and the correct tRNA for this amino acid and joins the two together with a covalent bond at the unique binding site
How many different enzymes are there which are used in charging tRNA?
20- one for each different amino acid
What is translation?
Synthesis of proteins by ribosomes using mRNA as a set of instructions
What do ribosomes contain?
Ribosomal RNA and proteins
What are some functional gene products which aren’t proteins?
tRNA and rRNA
What are the two parts of the ribosome?
Small and large subunit
What binds at the small subunit?
mRNA
What binds at the large subunit?
tRNA
How many sites are in a ribosome?
3
What are the 3 sites in the ribosome?
A, P and E
What happens at the A site?
Charged tRNA enters the ribosome
What is located at the P site?
tRNA with a growing chain of amino acids
What is above the P site and why?
A tunnel so that the amino acids can move out of the ribosome
What happens at the E site?
Empty tRNA leaves the ribosome
Where can ribosomes be found?
Bound to the rough ER and free in the cytosol
What do ribosomes bound to the rough ER do?
synthesise proteins that are used within the plasma membrane or are exocytosed from the cell
What do ribosomes free in the cytosol do?
Synthesise proteins that are released into the cytosol and used within the cell
What are the stages of translation?
Initiation, Elongation and Termination
What do all three stages of translation require?
Energy
What is the first part of initiation?
A specific initiation tRNA carrying methionine binds to the small ribosomal subunit
What happens once the initiation tRNA has bonded to the small ribosomal subunit?
The small ribosomal subunit/initiator tRNA identifies the 5’ G cap and attaches to the mRNA
What happens once the ribosomal subunit/initiator tRNA identifies the 5’ G cap and attaches to the mRNA?
The small ribosomal subunit/initiator tRNA complex moves along the mRNA in the 3’ to 5’ direction until it finds the initiation AUG codon
What happens once the initiation codon has been found?
The complex stops with the initiator tRNA carrying the first methionine positioned in the P site
What happens once the methionine is positioned in the P site?
The large ribosomal subunit attaches
What is the first part of elongation?
The next codon is read and a charged tRNA with an anticodon complementary to the A site codon lands in the A site
What happens after the next codon is read and a charged tRNA with an anticodon complementary to the A site codon lands in the A site?
Two things happen at the same time:
- The ribosome will break the bond that binds the amino acid to the tRNA in the P site, transfer the amino acid to the newly arrived amino acid (attached to the tRNA in the A site) and form a peptide bond between them
- While the tRNA’s are bound to the mRNA (in the P and A sites), the ribosome moves three nucleotides down the mRNA
What is in the A site in the first part of the second step of elongation?
tRNA with growing amino acid
What is in the P site in the first part of the second step of elongation?
empty/uncharged tRNA
What is in the P site in the second part of the second step of elongation?
tRNA with the growing chain of amino acids so that the amino acid chain can exit the tunnel above (it was in the A site)
What is in the E site in the second part of the second step of elongation?
Uncharged tRNA which was originally in the P site
What is the 3rd step of elongation?
In the E site the uncharged tRNA detaches from its anticodon and is released
What is the last step of elongation?
A new charged tRNA with an anticodon complementary to the next A site codon enters the ribosome at the A sire and the elongation process repeats itself
What is the first step on termination?
When the ribosome reaches a stop codon, a protein called release factor enters the A site
What is the second step of termination?
Release factor breaks the bond between the P site tRNA (using water) and the final amino acid. This causes the polypeptide chain to detach from its tRNA and the newly made polypeptide is released
What is the last step of elongation?
Small and large ribosomal subunits dissociate from the mRNA and each other
What is genotype?
Sets of genes and/or DNA combinations that are responsible for a particular trait
When is genotype determined?
At the DNA level
What is phenotype?
Physical expression or characteristics of a genotype (traits or characteristics of an organism that can be observed)
What is phenotype defined by?
What happens at the protein level together with the environmental influence
What can be said about genotype and phenotype?
What happens at the DNA level is reflected at the protein level and genetic variation commonly leads to phenotypic variation
