Transcription and translation Flashcards
How many letters code for an amino acid ?
Every three letters
mRNA
Messenger RNA is the carrier of information from DNA to the protein-synthesizing machinery (ribosome)
tRNA
Transfer RNA serves as adapter molecule in protein synthesis, translates mRNA to amino acids. Involved in translation
rRNA
Ribosomal RNA plays a structural and catalytic role in ribosomes
Exons
expressed regions
Introns
Intervening regions - noncoding regions
Describe transcription step 1
The production of pre-mRNA with the introns removed
What enzyme is responsible for transcription
RNA polymerase
Describe transcription step 2
The enzyme binds to the gene at the promoter region. In eukaryotes binding is mediated by transcription factors. The completed assembly of transcription factors and enzyme is called the transcription initiation complex.
Describe transcription step 3
The RNA polymerase unwinds the two strands of DNA and initiates RNA synthesis. The template strand is used this dictates the ordering of the nucleotides. The mRNA molecule is complimentary to the template strand. After the “transcription unit” has been transcribed the DNA double helix is reformed.
describe transcription step 4
Transcription proceeds until the RNA polymerase reaches a terminator sequence. The RNA polymerase detaches from the DNA immediately in prokaryotes and shortly after in eukaryotes
Post transcriptional modifications
The cell takes a moment to see if there is any introns left
What does RNA processing in eukaryotes include?
RNA splicing, addition of a Poly A Tail on the 3’ end, addition of a 5’ cap to the 5’ end
What is RNA splicing?
A cut and paste job where introns are removed from between exons
what is a Poly A Tail?
Many adenine nucleotides
What is a 5’ cap?
A modified Guanine nucleotide
What do the Poly A Tail and 5’ cap prevent?
they prevent the degradation of the mRNA ends and helps the ribosome attach to it in translation
Where does translation occur?
the ribosomes
what is the structure of tRNA ?
tRNA molecules are single stranded and folded, at one end there is a codon and at the other end there is an amino acid attachment site. the structure of tRNA fits its function
What are the three stages of translation?
Initiation, elongation and termination
Describe initiation of translation
Ribosome binds to the mRNA molecule. At the start of the mRNA molecule the start codon (AUG) signals the start of translation. An initiator codon binds to the start codon. The initiator codon is in the P-site, allowing the next tRNA to bind to the next codon.
What does the initiator codon carry?
the amino acid methionine
Describe elongation
Amino acids are added one by one to the first amino acid. The mRNA codon in the A site if the ribosome bonds with the complimentary anticodon of a tRNA molecule. The amino acid at the P site forms a peptide bond with the amino acid at the A site. This polypeptide separates from the P site and attaches itself to the tRNA molecule in the A site. The tRNA molecule now translocates to the P site. The amino acid in the P site moves to E site and out of the ribosome. This process continues until the polypeptide is built up.
Describe termination of translation
Elongation continues until a stop codon is reached. A protein called a release factor binds directly to the stop codon and causes the addition of a water molecules instead of an amino acid to the polypeptide. This frees the polypeptide from the ribosome.
What are ribosomes three bonding sites for tRNA molecules?
A,P,E
What are polyribosomes
Many ribosomes bind one after another
What are mutations ?
Changes in the genetic material of a cell
What are point mutations?
Changes in a small number of base pairs in a single gene
Base pair subsitutions?
Replacement of one DNA nucleotide with another
what is a missense mutation?
codes for an amino acid just not the right one
what is a nonsense mutation?
a codon which codes for no amino acid
Describe insertion and deletion mutations
The insertion or deletion of a base pair in the DNA will cause an interruption in the reading frame.
What is a frameshift mutation?
It produces a protein which is almost always nonfuctional
What is an example of a point mutation?
sickle cell disease
