Transcription and Translation

Question 1

What is the idea of the central dogma of molecular biology?

Answer 1

DNA makes RNA makes protein.

RNA can be converted back to DNA.

Question 2

How does a DNA sequence code for a protein?

Answer 2

• The DNA sequence is linearly related to the protein sequence.
• The DNA sequence is converted into a mRNA which carries the code of the protein.
• DNA has 4 bases which are used to made codons

Question 3

Explain what codons are

Answer 3

They code for individual amino acids. Each codon specifies one particular amino acid, this is referred to as the genetic code.

Question 4

Explain how the genetic code is degenerate and which 2 amino acids are not encoded for by more than one codon?

Answer 4

Same amino acid can be coded for by more than 1 codon, 64 combinations.

Tryptophan and methionine

Question 5

What is the importance of the reading frame?

Answer 5

Depending on where the beginning of the chain is, we can read it differently and produce different amino acids.

Question 6

Gice details on rRNA and mRNA

Answer 6

rRNA - largest in size and most abundant pf the 3 classes of RNA, important structural components of ribosomes, the site of translation/protein synthesis

mRNA - carries genetic information from the DNA to the ribosome, vary considerably in length, mRNA in eukaryotes has a long polyA tract at the 3’ end which aids in their isolation.

Question 7

Where is the open reading frame found?

Answer 7

Between the stop and start codon

Question 8

Give details on tRNA

Answer 8

• smallest class of RNA’s
• cloverleaf structure (due to internal base pairing)
• transfers amino acids from the cytoplasms amino acids pool to a ribosome
• the interpreter of the codons contains in a mRNA
• three base sequences called the anitcodon binds to complementary codon on the mRNA by hydrogen bonding

Question 9

Explain the mechanism of transcription

Answer 9

A small portion of the DNA double helix unwinds to expose the bases on each strand
One of the 2 stands of the DNA acts as a template
Transcription starts
The RNA transcript is elongated one nucleotide at a time and is complementary to the template strand
As the RNA chain is made, the DNA helix reforms behind it, and the RNA is released as a single strand
RNA molecules are quite small compared to DNA, usually a few hundred to thousand bases long

Question 10

Outline transcription in bacteria

Answer 10

Carried out by RNA polymerase to create a complementary copy of the template DNA.
The process involves 3 main steps (initiation, elongation and termination).

A polycistonic mRNA that contains information for more than one peptide is produced.
They have circular single chromosomes. This means the mRNA contains more than one gene.

The mRNA starts to be translated as soon as it is made, multiple peptides maybe made from the same mRNA strand.

Question 11

Give details on eukaryotic transcription

Answer 11

More complex than in bacteria
RNA is transcribed in the nucleus
Transcription requires the formation of a transcription initiation complex. The RNA pol 2 enzyme copies DNA into RNA with the help of transcription factors.
mRNA is synthesised and then processed (key difference to bacteria and in bacteria there is no processing - as soon as it is transcribed, it is translated)
Capped at the 3’end
Poly-adenylate at the 3’end
Spliced to remove all introns (the non-coding past of the gene)
Selective removal of some exons may also occur

Question 12

Explain the role of capping

Answer 12

Addition of a poly A tail to the 3’end of mRNA
Carried out by a number of proteins which bind to specific sequences in the 3’ un-translated region of the gene
Results in a long polyA which stabilises mRNA and aids in its transport

The 5’end of the RNA is chemically modified by the addition of a “cap”
A 7-methyl guanosine (M7G) residue is added in the reverse orientation compared with the normal 3’-5’bridge, giving a 5’-5’ triphosphate bridge.
Protects the RNA from exonucleases (decapped RNAs are rapidly degraded)
Facilitates transport of mRNA from the nucleus to the cytoplasm
plays a role in the attachment of the mRNA to the 40S subunit of ribosomal RNA

Question 13

Explain what polyadenylation is

Answer 13

• addition of a poly A tail to the 3’ end of mRNA
• carried out by a number of proteins which bind to specific sequences in the 3’ un-translated region of the gene
• results in a long polyA tail which stabilises mRNA and aids in its transports

Question 14

What is the importance of splicing?

What has to occur once the splicing is done?

Answer 14

Excision and removal of introns (non coding regions)
Splicing together of axons (coding regions)
All introns needs to be removed
Transesterification removes this

The exons need to be joined together

Question 15

How many approx exons are there per gene and how many introns?

Answer 15

8. 8 exons

7. 8 introns

Question 16

What part of the DNA is transcribed?

Answer 16

The entire gene (introns and exons).
The introns are then removed and exons have to be joined.
Some exons are selectively removes (differential splicing).

Question 17

Explain the importance of differential splicing

Answer 17

mRNA undergoes this giving rise to a series of mRNA containing different exon and each specialising a different protein.

Question 18

What are the differences between prokaryotes and eukaryotes?

Answer 18

Eukaryotic RNA is processes during transcription. In bacteria, no processing occurs and translation often begins before translation is complete.

Eukaryotic RNA:

• Have a cap added to the 5’ end
• Most are poly-adenylate by the addition to the 3’ end.
• Many contain introns so undergo splicing
• A few are subject to RNA editing
• Involves a large number of polypeptides

Question 19

Where does transcription take place?

Answer 19

In the cytosol

Question 20

Explain the shape of ribosomes

Answer 20

Made of 2 subunits (small and large).
prokaryotes - 70S
eukaryotes - 80S
Each ribosome has three binding sites for tRNA in its large subunit.

Question 21

What are the 3 sites on a ribosome?

Answer 21

A site (aminoacyl) - holds the tRNA carrying the next amino acid to be added to the chain.

P site (peptidyl) - holds the tRNA carrying the growing polypeptide

E site (exit) - discharged tRNAs leave the ribosome through this site. The amino acids are brought to the ribosomes by tRNA molcules.

Question 22

What do ribosome catalyse and how?

Answer 22

Ribosomes bring mRNA together with charged tRNAs and catalyse coupling of amino acids into protein according to the sequence specified by the mRNA.

Question 23

What enzyme charge tRNA with the correct amino acid?

What provides the energy to do this?

Answer 23

Amino acyl tRNA synthetases

Hydrolysis of ATP

Question 24

Explain the 3 stages of translation

Answer 24

1. Initiation = the ribosome is bound to the specific initiation site on the mRNA. This is the start codon which is always the same. The initiator tRNS in annealed to the initiator codon and bound to the ribosome.
2. Elongation = consists of joining amino acids together to the growing polypeptide chain according to the sequence specified by the message. Incorporation of each amino acid occurs by the same mechanism. The same steps are repeated over and over until the stop codon is met.
3. Termination = termination codon gives the signal for the third and last stage of protein synthesis, in which the newly made peptide is released from the ribosome.

Question 25

How many proteins does prokaryotic mRNA code for?

eukaryotic?

Answer 25

more than one

one

Question 26

What allowed antibiotic development?

Answer 26

Differences in translation between prokaryotes and eukaryotes.

Question 27

What is peptidyltransferase and what would happen without it?

Answer 27

Forms peptide bonds between the amino acids.

Translation would be blocked without it.