2.3 Nucleotides

Question 1

Describe / draw the structure of a DNA nucleotide.

Answer 1

A DNA nucleotide is composed of a phosphate group, pentose sugar with a H at 2’ and a nitrogenous base.

Question 2

What are the 4 bases in DNA?

Answer 2

Adenine, Thymine, Guanine and Cytosine.

Question 3

What are the 4 bases in RNA?

Answer 3

Adenine, Uracil, Guanine, Cytosine.

Question 4

How does an RNA nucleotide differ to a DNA nucleotide?

Answer 4

The 2’ carbon in an RNA nucleotide has a hydroxyl group whereas DNA nucleotides just have H.

Question 5

What bases have a pyrimidine structure (single ring) and what bases have a purine structure (double ring) ?

Answer 5

Adenine and guanine are purine molecules and thymine, cytosine and uracil are pyrimidine molecules.

Question 6

DNA and RNA are polynucleotides. What bonds form between the monomers of these polymers?

Answer 6

A condensation reaction between two nucleotides forms a phosphodiester bond . The phosphate group coming from C 5’ on one nucleotide has a hydroxyl group that reacts with a hydroxyl from C 3’ on another nucleotide. This leaves H2O as a product.

Question 7

Describe the structure of Adenosine Triphosphate.

Answer 7

Adenosine Triphosphate is a phosphorylated nucleoside (Nitrogenous base + ribose sugar) that has 3 phosphate groups bound to the sugar molecule.

Question 8

Describe the role of ATP.

Answer 8

The hydrolysis of ATP provides the energy required to move macromolecules in and out of cells and anabolic (building) reactions. In animals ATP is used for muscle contractions and the conduction of nerve impulses.

Question 9

Describe the structure of a double helix.

Answer 9

A DNA molecule is a double helix has two strands which are said to run antiparallel (5’ to 3’ and 3’ to 5’) as the bases in nucleotides alternate for H bonding.

Question 10

How many H bonds between the DNA base pairs adenine / thymine + uracil and cytosine / guanine?

Answer 10

There are two bonds between A-T/U and three between C-G.

Question 11

Outline the steps of the marmur preparation to form DNA precipitates.

Answer 11

•Add cut up fruit to a mixture of detergent and water, heat to 60’C (this breaks the phospholipid bilayer)
•Cool mixture
•Blend mixture for a few seconds (destroying cell wall)
•Filter mixture to remove debris
•Add protease enzyme
•Add ice cold ethanol (nucleic acids are insoluble in ethanol so precipitate)

Question 12

Outline the process of semi conservative replication.

Answer 12

•DNA helicase destroys the H bonds between base pairs, unwinding the helix
•Each strand now acts as a template strand and free nucleotides join with DNA polymerase as a catalyst

Question 13

How may mutations occur in DNA replication?

Answer 13

DNA replication is very fast and accurate but sometimes incorrect bases / extra bases are added or left out

Question 14

mRNA is formed in the nucleus via transcription. Outline the process of transcription

Answer 14

In the nucleus, DNA helicase unwinds the two strands and exposes a gene
Free RNA nucleotides pair up to the target template strand and phosphodiester bonds form between the sugar phosphate groups (catalysed by RNA polymerase)
The bonds between the mRNA and DNA now break, the mRNA leaves the nucleus and the DNA reforms.

Question 15

RNA nucleotides only pair with bases on one side of the DNA molecule : the template strand. In what way does RNA polymerase form mRNA on this strand ?

Answer 15

RNA polymerase moves along the template strand on the 3’ to 5’ direction.

Question 16

What base is not present in mRNA ? What is it replaced with ?

Answer 16

Thymine is replaced with Uracil in mRNA. Uracil bonds to adesine and is a pyrimidine.

Question 17

Explain how tRNA helps code for proteins in ribosomes.

Answer 17

tRNA acts as a physical link between mRNA and amino acids. They bind to a specific amino acid from the cytoplasm and also to a codon on mRNA (via anticodon) and stay in place until peptide bonds can be formed.

Question 18

How many tRNA molecules can be present in a ribosome at one time?

Answer 18

Two

Question 19

What is a start and stop codon ?

Answer 19

Start codons (methionine) start a peptide chain and stop codons terminate it.

Question 20

What are the pentose sugars in DNA and RNA.

Answer 20

•Dna - Deoxyribose
•RNA - Ribose

Question 21

Identify 3 features of the genetic code.

Answer 21

Non overlapping - each triplet is read only once
Degenerative - more than one triplet codes for each amino acid
Universal - all organisms use same amino acids and triplets

Question 22

Translation is the process of synthesising polypeptides. Outline this process.

Answer 22

Ribosome moves along mRNA until it reaches the start codon
tRNA, with amino acids, bind to first two triplets via the anticodon and peptide bonds are formed (energy comes from hydrolysis of ATP)
Only two tRNA can be on mRNA at once, so once peptide bond formed the first tRNA leaves into the nucleus
This process repeats until a stop codon.

Question 23

What happens to tRNA once it deposits its amino acid?

Answer 23

It goes into the nucleus and is bonded with another of the same amino acid via hydrolysis of ATP