12. Nucleic acids

Question 1

The basic unit of nucleic acids is a nucleotide. What is a nucleotide made of?

Answer 1

1. A 5C sugar with a 5 membered ring (ribose/deoxyribose)
2. Phosphate group
3. Nitrogenous base – adenine / cytosine / guanine / thymine (uracil for RNA)

Question 2

What structures do single stranded nucleic acids form?

Answer 2

Stem-loop

Question 3

In the Watson-crick model of DNA,
what kinds of bonds hold the DNA double helix together?

Answer 3

• ladder/ backbone “ sugar-phosphate backbone –> covalent bonding between 5’ carbon on one sugar molecule and 3’phosphate group of another sugar molecule
• between complementary base pairs of nucleotides : hydrogen bonds
• between “rungs” of the ladder –> each layer of nucleotide base pairs –> hydrogen bond

Question 4

What is the difference between positive supercoiling and negative supercoiling?

Answer 4

• Positive supercoiling – condenses DNA effectively but harder to separate
• Negative supercoiling – prepares DNA for processes requiring separation (e.g. replication)

Question 5

DNA REPLICATION

How is the leading strand synthesised?

Answer 5

• Primer added the primase marks the start of DNA construction
• DNA polymerase add nucleotide bases to each new strand
• Primers are removed by exonuclease and DNA polymerase fills in the gaps
• DNA ligase seals the DNA strand

Question 6

which direction is new dna synthesised , from outer wider part of replication fork into the inner part where the fork just opens or from inside –> outside

Answer 6

outside –> inside

Question 7

Why lagging strand synthesized in small fragments (Okazaki fragments)?

Answer 7

The strand is in the opposite direction from direction of synthesis (outside –> inside). Thus , to synthesize from 5’ to 3’ end, DNA polymerase must work backwards by adding primers from abit away from 3’ end —> add nucleotides downwards —> add another primer further up —> synthesise downwards —> repeat and work its way up to the 5’ end since can only synthesize in 5’ –> 3’ direction

Question 9

What is the start codon for translation of RNA to proteins?

Answer 9

AUG at 5’ end

Question 10

What specific codons terminate polypeptide chain? [3]

What are these codons read by

Answer 10

UAA, UAG and UGA

Read by release factors

Question 11

What is the enzyme that splices exons from pre mRNA molecules?

Answer 11

Splicosomes

Question 12

In ribosome, how big is the LARGE subunit, and how many RNA molecules are there? What are the sizes of each RNA molecule?

Answer 12

Large subunit : 60S
- S : Svedberg units

Contains 3 RNA molecules
- 28S, 5.8S and 5S

Question 13

In ribosome, how big is the SMALL subunit, and how many RNA molecules are there? What are the sizes of each RNA molecule?

Answer 13

Small subunit : 40S
- S : Svedberg units

Contains 1 RNA molecule
- 18S

Question 14

Ribosomes are free in the cytoplasm, or are attached to the Rough Endoplasmic Reticulum (RER).

Where do the proteins go after being synthesized in cytoplasm / RER?

Answer 14

Proteins synthesized in cytoplasm
- remain within the cell in the cytoplasm OR
- directed to organelles bounded by a double membrane (nucleus / mitochondria)

Proteins synthesized in RER
- packed into Golgi apparatus for further processing and sorting, and packed into vesicles–> proteins modified –> for specific functions

• transported to cell membrane for secretion out of cell OR
• incorporated into the cell membrane