Nucleic acids

Question 1

DNA meaning, structure and function

Answer 1

Deoxyribonucleic acid holds genetic information.

Consists of a phosphate group, nitrogenous base and a pentose sugar (deoxyribose)

Question 2

RNA meaning, structure and function

Answer 2

Ribonucleic acid transfers genetic information from DNA to ribosomes

Consists of a phosphate group, nitrogenous base and a pentose sugar (ribose)

Question 3

what are ribosomes formed from

Answer 3

RNA and proteins

Question 4

Bases in DNA

Answer 4

Adenine-Thymine,
Guanine-Cytosine

Question 5

Bases in RNA

Answer 5

Adenine-Uracil,
Guanine-Cytosine

Question 6

how do nucleotides form nucleic acids?

Answer 6

DNA or RNA nucleotides are joined together by condensation reactions

Forms phosphodiester bonds

Question 7

DNA structure

Answer 7

2 strands joined in anti-parallel, held together by hydrogen bonds (between specific complementary base pairs; A-T, G-C)

Double helix

Question 8

RNA structure

Answer 8

Single RNA polynucleotide strand

Question 9

Differences between RNA and DNA

Answer 9

DNA molecules are double-stranded (double helix), whereas RNA molecules are single-stranded

DNA is longer whereas RNA is shorter

Question 10

Structure of DNA related to its functions (hydrogen bonds)

Answer 10

Weak hydrogen bonds between bases: can be unzipped easily for replication

Many hydrogen bonds between bases: stable/strong molecule

Question 11

Structure of DNA related to its functions (helical)

Answer 11

Double helix with a sugar-phosphate backbone: protects bases and hydrogen bonds

Double helix (coiled): compact

Long molecule: store lots of genetic information that codes for polypeptides

Question 12

Structure of DNA related to its functions (shape)

Answer 12

Complementary base pairing: accurate replication

Double-stranded: both strands can act as templates for semi-conservative replication

Question 13

Process of DNA replication

Answer 13

DNA helicase breaks hydrogen bonds between bases and unwinds the double helix.

Results in two strands which both act as templates.

Free-floating DNA nucleotides are attracted to the exposed bases via specific complementary base pairing

Hydrogen bonds form: adenine-thymine, guanine-cytosine

DNA polymerase joins adjacent nucleotides on new strand by condensation, which forms phosphodiester bonds (sugar-phosphate backbone)

Replication is semi-conservative: each new strand formed contains one template strand and one new strand.

Ensures genetic continuity between generations of cells.

Question 14

How does DNA polymerase join adjacent nucleotides?

Answer 14

DNA polymerase moves in opposite directions along the DNA strands (DNA has anti-parallel strands)

DNA polymerase is an enzyme with a specific active site which can only bind to substrates with complementary shapes.

It can only bind to and add nucleotides to the phosphate (3’) end of the developing strand.

Works in a 5’ to 3’ direction

5’ carbon has a phosphate group attached

3’ carbon has a hydroxyl (-OH) group attached

Question 15

What is ATP

Answer 15

Adenosine triphosphate

Ribose, a molecule of adenine and 3 phosphate groups

Question 16

What is ADP

Answer 16

Adenosine diphosphate

Ribose, a molecule of adenine and 2 phosphate groups

Question 17

ATP hydrolysis
(ATP -> ADP + Pi)

Answer 17

Catalysed by the enzyme ATP hydrolase

Can be coupled with energy-requiring reactions to provide energy for active transport, protein synthesis, etc (energy is released when the bonds between inorganic phosphate groups are broken)

The inorganic phosphate released can be used to phosphorylate other compounds eg glucose, often making them more reactive (ie lowers activation energy)

Question 18

ATP/ADP condensation (ADP + Pi -> ATP)

Answer 18

Catalysed by the enzyme ATP synthase

Happens during respiration or photosynthesis

Also called phosphorylation of ADP

Formation of a water molecule

Question 19

Properties of ATP
(that makes it a suitable immediate source of energy; ATP cannot be stored)

Answer 19

ATP releases energy in small, manageable amounts (no energy is wasted)

Only one bond is hydrolysed (single reaction) to release energy (which is why energy release is immediate)

Question 20

Evidence for semi-conservative replication (Meselson and Stahl) process

Answer 20

Bacteria is grown in a nutrient containing heavy nitrogen (15N) for several generations

Nitrogen is therefore incorporated into bacterial DNA bases

Bacteria then transferred to a nutrient solution containing light nitrogen (14N) and allowed to grow and divide twice

During this process, DNA from different samples of bacteria was extracted, suspended in a solution in separate tubes and spun in a centrifuge

Question 21

Evidence for semi-conservative replication (Meselson and Stahl)
results

Answer 21

Sample 1: DNA from bacteria grown for several generations in a nutrient solution containing 15N. DNA molecules contain two heavy strands

Sample 2: DNA from bacteria grown originally in a nutrient solution containing 15N, then transferred for one division to a solution containing 14N. DNA molecules contain 1 original heavy strand, and one new light strand

Sample 3: DNA from bacteria grown originally in nutrient solution containing 15N, then transferred for two divisions to a solution containing 14N. 50% DNA molecules contained 1 heavy and 1 light strand, other 50% contained both light strands