L8 - DNA replication, transcription and translation

Question 1

What are facts about DNA replication?

Answer 1

• copying of DNA to create a new identical strain
• has to be copied before a cell divides
• copied during synthesis phase of interphase
• new cells will need identical DNA strands
• happens at the nucleus of eukaryotes

Question 2

What is the first step of DNA replication?

Answer 2

• 2 strands are open forming replication forks (Y-shaped)
• using DNA helicase to break the H bonds between the bases

Question 3

What happens as the DNA strands open at the origin?

Answer 3

replication bubbles form
- prokaryotes have a single bubble
- eukaryotic chromosomes have many bubbles
= the bubbles end up joining together

Question 4

How are the strands kept separate?

Answer 4

enzyme topoisomerase
single-strand binding proteins attach
- binds further down, chops a strand of DNA which releases stress
- keeps it untwisted

Question 5

What binds before new DNA strands can form?

Answer 5

primase
- synthesises RNA primers
- allows DNA polymerase to bind

Question 6

What way does DNA polymerase add nucleotides?

Answer 6

• 5’ to 3’ direction
• antiparallel

Question 7

What is the leading strand?

Answer 7

The strand that forms a continuous strand of DNA from the point of origin toward the opening replication fork

Question 8

What is the lagging strand?

Answer 8

strand that forms short small sequences
- as it has to be formed 5’ to 3’
- short strands are called okazaki fragments

Question 9

What enzyme joins the okazaki fragments together?

Answer 9

DNA ligase

Question 10

Features of proofreading new DNA

Answer 10

• DNA polymerase makes about 1 in10,000 base pairing errors
• enzymes proofread
• error becomes 1 in 1 billion

Question 11

How does the enzyme proofread DNA?

Answer 11

• incorporates of incorrect nucleotides at 3’ end
• exonuclease removes mis-paired bases
• continues of DNA replication

Question 12

What is the semi-conservative model of replication?

Answer 12

• 2 strands of parental molecule separate
• each acts as a template for a new complementary strand
• New DNA consists of 1 parental and 1 new strand of DNA

Question 13

What is the first function of DNA?

Answer 13

store genetic information
- replication passes the information to daughter cells and new organisms

Question 14

What is the second function of DNA?

Answer 14

Direct synthesis of proteins
- determines cell structure and function

Question 15

What are the 2 stages of synthesis?

Answer 15

• transcription
• translation

Question 16

What is gene expression?

Answer 16

the transformation of DNA information into functional molecules

Question 17

What is transcription?

Answer 17

production of a section of DNA carrying information related to one or more genes
- as mRNA

Question 18

Key points for transcription:

Answer 18

• RNA polymerase separates strands
• doesn’t need a primer
• doesn’t need proof reading
• mRNA synthesis starts at many places and stops at predetermined places
• when mRNA molecule complete, it’s released as a single stranded molecule

Question 19

What is the Eukaryotic promoter site?

Answer 19

section of DNA that acts as a start signal
- CAAT box (sometimes present)
- TATA box (Hogness box)

Question 20

What is the initiation step in transcription?

Answer 20

• polymerase binds to promoter sequence in duplex DNA”closed complex”
• Polymerase melts DNA near transcription site (transcription bubble) “open complex”
• polymerase catalyses phosphodiester linkage of 2 initials rNTPs

Question 20

What is the elongation step in transcription?

Answer 20

• polymerase advances 3’ to 5’
• melting duplex DNA and adding rNTPs to growing RNA

Question 21

What is the termination step in transcription?

Answer 21

at the transcription stop site, polymerase releases completed RNA and dissociated from DNA

Question 22

What is the stop signal in transcription?

Answer 22

self complementary sequence in the mRNA forms hairpin loop
- recognised by RNA polymerase
= stops synthesis

Question 23

What is translation?

Answer 23

using the sequence of information on the MRNA molecule to construct a protein molecule by assembling a/a in the correct order
- takes place in the ribosome
- involved tRNA

Question 24

What are the stop/nonsense codons?

Answer 24

UAA
UAG
UGA

Question 25

What is the start codon?

Answer 25

AUG

Question 26

Features of the triplet code:

Answer 26

• degenerate: some a/a are coded for by more than one codon
• universal: in all organisms (except a few exceptions)

Question 27

What is the structure of tRNA?

Answer 27

• all fold into 4 base paired stems and 3 loops
• the CCA sequence at the 3’ end is common
• some of the CGAU residues are modified

Question 28

What is aminoacyl-tRNA?

Answer 28

aminoacyl-tRNA synthetase (enzyme)
- links each tRNA with its appropriate a/a

Question 29

What is the enzyme that forms the peptide bond?

Answer 29

peptidyl transferase
- brought about in ribosomes

Question 30

What are ribosomes like in Eukaryotes?

Answer 30

Made of a num of dif rRNA
- 2 subunits: 60S and 40S subunits
- S: rate of sedimentation during centrifugation

Question 31

What is the initiation step in translation?

Answer 31

• starts at AUG codon closest to 5’ end
• initiator tRNA binds to small subunit of the ribosome (methionyl-t-RNA^Met)
• binds to mRNA, moves alond to find AUG
• initiation factors dissociate, large rRNA binds
• 2 binding sites peptidyl site (P) or aminoacyl site (A)
• second a/a binds to A

Question 32

What is the elongation process in translation?

Answer 32

• tRNA bearing growing peptide is at the P site
• next a/a moves into A site
• peptide bond formed by the enzyme peptidyl transferase
• complex moves along mRNA, leaves A site for the next a/a

Question 33

What is the termination step in translation?

Answer 33

• moves along mRNA until stop codon reached
• release factor bings in the A site
• has no a/a, so terminates chain = COOH
• stimulates separation of the subunits of the ribosome