Transcription and Translation Quiz Flashcards
Compare RNA and DNA
Outline DNA trasncription in terms of the formation of an RNA strand
RNA polymerase separates the DNA strands and synthesises a complementary RNA copy from one of the DNA strands
It does this by covalently bonding ribonucleoside triphosphates that align opposite their exposed complementary partner (using the energy from the cleavage of the additional phosphate groups to join them together)
Once the RNA sequence has been synthesised, RNA polymerase will detach from the DNA molecule and the double helix will reform
The sequence of DNA that is transcribed into RNA is called a gene
Transcription occurs in the nucleus (where the DNA is) and, once made, the mRNA moves to the cytoplasm (where translation can occur)
Describe gentic code in terms of codons
Codons are a triplet of bases which encodes a particular amino acid
As there are four bases, there are 64 different codon combinations (4 x 4 x 4 = 64)
The order of the codons determines the amino acid sequence for a protein
The coding region always starts with a START codon (AUG) and terminates with a STOP codon
It is degenerate - there are only 20 amino acids but 64 codons, so more than one codon may code for the same amino acid (this allows for silent mutations whereby a change in the DNA sequence does not affect the polypeptide sequence)
What direction is transcription carried out
mRNA is composed of nucleotides based on Ribose sugar and four bases Uracil, , Adenine, Guanine and Cytosine.
The 5’ end of the nucleotide is added to the 3’ of the already existing mRNA chain.
Distinguish between sense and antisense strands
The sense strand has the same base sequence as the transcribed mRNA except that the base thymine is replaced by the base uracil.
The anti-sense strand acts as the template for the transcription of mRNA.
Explain Transcription in Prokaryotes
Transcription is the process by which a DNA sequence (gene) is copied into a complementary RNA sequence and involves a number of steps:
RNA polymerase binds to the promoter and causes the unwinding and separation of the DNA strands
Nucleoside triphosphates (NTPs) bind to their complementary bases on the antisense strand (uracil pairs with adenine, cytosine pairs with guanine)
RNA polymerase covalently binds the NTPs together in a reaction that involves the release of two phosphates to gain the required energy
RNA polymerase synthesises an RNA strand in a 5’ - 3’ direction until it reaches the terminator
At the terminator, RNA polymerase and the newly formed RNA strand both detach from the antisense template, and the DNA rewinds
Many RNA polymerase enzymes can transcribe a DNA sequence sequentially, producing a large number of transcripts
Post-transcriptional modification is necessary in eukaryotes
Explain process of Translation
Translation is the process of protein synthesis in which the genetic information encoded in mRNA is translated into a sequence of amino acids in a polypeptide chain
Ribosomes bind to mRNA in the cell’s cytoplasm and move along the mRNA molecule in a 5’ - 3’ direction until it reaches a start codon (AUG)
Anticodons on tRNA molecules align opposite appropriate codons according to complementary base pairing (e.g. UAC will align with AUG)
Each tRNA molecule carries a specific amino acid (according to the genetic code)
Ribosomes catalyse the formation of peptide bonds between adjacent amino acids (via a condensation reaction)
The ribosome moves along the mRNA molecule synthesising a polypeptide chain until it reaches a stop codon, at this point translation stops and the polypeptide chain is released
Relationship between one gene and one polypeptide
A gene is a sequence of DNA which encodes a polypeptide sequence
A gene sequence is converted into a polypeptide sequence via the processes of transcription (making an mRNA transcript) and translation (polypeptide synthesis)
Translation uses tRNA molecules and ribosomes to join amino acids into a polypeptide chain according to the mRNA sequence (as read in codons)
tRNA
shape of each tRNA is different.
shape of the tRNA is defined by the loop and the helical sections.
shape of the tRNA selects a specific enzyme (aminoacyl-tRNA synthetase).
the enzyme adds a specific amino acid to the CCA base sequence (at 3’ end of the tRNA) this requires ATP (energy).
each amino acid has one or more tRNA molecules this again is an example of a degenerate code.
Structure of RIbosomes
Ribosomes are made of protein (for stability) and ribosomal RNA (rRNA - for catalytic activity)
They consist of two subunits:
The small subunit contains an mRNA binding site
The large subunit contains three tRNA binding sites - an aminacyl (A) site, a peptidyl (P) site and an exit (E) site
Ribosomes can be either found freely in the cytosol or bound to the rough ER (in eukaryotes) Ribosomes differ in size in eukaryotes and prokaryotes (eukaryotes = 80S ; prokaryotes = 70S)
Translation Process
Translation occurs in four main steps:
Initiation: Involves the assembly of an active ribosomal complex
Elongation: New amino acids are brought to the ribosome according to the codon sequence
Translocation: Amino acids are translocated to a growing polypeptide chain
Termination: At certain “stop” codons, translation is ended and the polypeptide is released
Translation Direction
Peptide Bond
Translation Process with Ribosome
The tRNA charged with Methionine has the anti-codon UAC. This is complementary to the start codon (mRNA) of AUG.
The small sub unit of the ribosome associates with the Methionine tRNA.
The small unit of the ribosome moves over the START codon.
The large unit of the ribosome moves over the mRNA.
There are three binding sites for tRNA on the large sub unit.
A-(Amino acid) is the position which the new tRNA codon-anticodon binds making sure that the correct amino acid is in position.
P-(Polypeptide) is the position in which the amino acid on the tRNA adds to the polypeptide.
E-( Exit) is the position the tRNA (without amino acid) locates and is the released from the ribosome to become re-activated.
Ribosomes
Free ribosomes in the cytoplasm are associated with the synthesis of proteins for internal use in the cell.
Ribosomes which are attached to the wall of the endoplasmic reticulum are associated with proteins which will ne placed into vesicles and secreted form the cell.
