DNA replication

Question 1

Primer strand

Answer 1

Strand being synthesised from template strand. Aka “growing strand”.
Nucleotides always added to the 3’ end of the strand, I.e polymerisation is in the 5’ to3’ direction.

Question 2

DNA polymerase exonuclease activity

Answer 2

The enzyme that synthesises new DNA in prokaryotes. The 3’ to 5’ exonuclease activity means the enzyme can remove the last nucleotide it has added. A part of the enzyme double checks every nucleotide as it is added, and if it is wrong, the enzyme will pause, move back slightly and use its exonuclease activity to remove the incorrect base before proceeding. Called proofreading. Not used in replication, only in DNA repair when damage has occurred.

Question 3

Okazaki fragments

Answer 3

Way of overcoming errors in replication. DNA pol follows after the replication fork and synthesised away from the fork in very short segments. DNA pol will synthesise a fragment until t hits the next one. Each fragment is about 1000bps, so the DNA pol never moves away from the fork took far. Although synthesis is not continuous on the lagging strand, the little gap between Okazaki fragments is easily filled by DNA pol 1 or DNA Ligase

Question 4

Mechanism of replication: initiation

Answer 4

DNA helicase begins to unwind a local region. It is localised to the right spot by initiator proteins which bind to the origin of replication.
Topoisomerases bind to dsDNA to continually release tortional stress. While single strand DNA binding proteins (SSB’s) bind to the ssDNA and keep the strands apart.
Short fragments of RNA synthesised by primate bind to either strand to act as primers- DNA cannot start from scratch but can only elongate existing nucleic acid.

Question 5

Mechanism of replication: elongation

Answer 5

DNA pol 3 starts its work by synthesising new DNA. It travels in the direction of the replication fork and synthesised in hen5’ to 3’ direction. The DNA pol 3 on the leading strand will polymerise continually while the DNA pol 3 on the lagging strand will synthesise in fragments

Question 6

Mechanism of replication: termination

Answer 6

RNA fragments on both strands (mostly lagging) will be excised by the exonuclease activity of DNA pol 1.
Little gal left between Okazaki fragments is filled by DNA pol 1
Final chemical join between 3’ hydroxyl and the 5’ phosphate group is catalysed by DNA Ligase.

Question 7

Centromeres - gene structure

Answer 7

DNA sequences that allow the attachment of spindle fibres

Question 8

Telomeres - gene structure

Answer 8

Tips of the chromatids. Capped at the ends of DNA that protect DNA in the chromosome

Question 9

Transposons - gene structure

Answer 9

DNA sequences that can jump from one region of the genome to another.

Question 10

Pseudogenes - gene structure

Answer 10

Genomic sequences similar to actual genes, but are non-functional

Question 11

Messenger RNA (mRNA)

Answer 11

Encodes the amino acid sequences specified by at least one gene.
Copied from DNA by base pairing.
Complementary sequence to its parent.
DNA strand which acts as the template has the complementary strand to its RNA. It is non-coding strand.
The non-template DNA strand has the same sequence as the RNA transcript and is called the coding strand.

Question 12

Transfer RNA (tRNA)

Answer 12

Bring amino acids to form protein.

Question 13

Ribosomal RNA(rRNA)

Answer 13

Make up he bulk of the ribosome- site of protein synthesis

Question 14

Micro RNA (miRNA)

Answer 14

Binds to mRNA and either blocks translation or cause degradation, hence acting as regulators.