Nucleotides and Nucleic Acids

Question 1

What are the two categories of nitrogenous bases

Answer 1

Purines- 2 carbon ring structures
Pyrimidines- 1 carbon ring structure

Question 2

Examples of purines

Answer 2

Adenine and guanine

Question 3

Examples of pyrimidines

Answer 3

Cytosine, thymine and uracil

Question 4

How are the polymers DNA and RNA made

Answer 4

nucleotides undergo condensation reactions forming phosphodiester bonds between the nucleotide monomers

Question 5

Features of a polynucleotide

Answer 5

phosphodiester bonds (strong covalent bonds) form between pentose sugar and phosphate of different nucleotides

Question 6

Features of ATP

Answer 6

Contains 3 phosphate ions
Essential for metabolism
Immediate source of energy
Composed of adenine, ribose and three inorganic phosphate groups
Made during respiration via a condensation reaction using ATP synthase

Question 7

Structure of DNA

Answer 7

Polymer forms double helix made of two anti parallel strands joined together by hydrogen bonds between the complementary bases on the two strands

Question 8

Function of DNA

Answer 8

Codes for the sequence of amino acids in the primary structure of a protein (which determines the final 3D structure and function of the protein)

Question 9

How does DNA structure relate to its function

Answer 9

Stable structure due to sugar phosphate backbone (covalent bonds) and the double helix
Double stranded so replication can occur using both strands as a template
Weak hydrogen bonds between bases for easy separation of the strands during replication
A large molecule that carries a lot of information
Complementary base pairing allows identical copies to be made

Question 10

Describe DNA precipitation

Answer 10

Homogenise the cell with detergent (this will break open the cells and cell membranes to release their contents)
Filter to remove large debris
Add salt to break hydrogen bonds between the DNA and water molecules
Add protease to digest the proteins associated with the DNA
Add ice cold ethanol to precipitate out the DNA from the solution, the DNA should appear as white strands

Question 11

What are the three types of RNA

Answer 11

mRNA (messenger RNA)
tRNA (transfer RNA)
rRNA (ribosomal RNA)

Question 12

Function of rRNA

Answer 12

Ribosomal RNA is the type of RNA that makes up the bulk of ribosomes (along with proteins)

Question 13

Structure/Function of mRNA

Answer 13

A copy of one gene from DNA
Created in the nucleus and leaves via the nuclear pore to carry the copy of the genetic code of one gene to a ribosome in the cytoplasm
Much shorter than DNA
Short lived
Single stranded, every 3 bases in the sequence codes for one specific amino acid (these 3 bases are called codons)

Question 14

Structure/Function of tRNA

Answer 14

Found in the cytoplasm
Is single stranded but folded to create a cloverleaf shape held in place by hydrogen bonds
Brings a specific amino acid to the ribosome (this is determined by the 3 bases found on the tRNA (anticodon) which are complementary to the 3 bases on mRNA (codon))

Question 15

Describe semi conservative DNA replication

Answer 15

One strand is conserved and a new strand is created
Occurs in the S-phase in interphase of the cell cycle
The enzyme that catalyses DNA replication is complimentary to the 3’ end and therefore can only attach at this location

Question 16

Stages of DNA replication

Answer 16

DNA helicase breaks the hydrogen bonds
Both strands acts as new templates for replication
Free floating nucleotides align opposite their complimentary bases on the template strand of DNA and hydrogen bonds then form between the base pairs
DNA polymerase joins adjacent DNA nucleotides together forming phosphodiester bonds between these nucleotides to create a new polymer chain of DNA

Question 17

Features (and their advantages) of the genetic code

Answer 17

Degenerate- Amino acids are coded for by more than one triplet of bases
(+) Some mutations will code for the same amino acid so there will be no affect

Universal- The same triplet of bases codes for the same amino acid in all organisms
(+)Able to remove human genes and insert it into a plasmid and the bacteria will make human insulin

Non-overlapping- Each base in a gene is only part of one triplet of bases that codes for one amino acid.
(+) If mutation occurs in one codon, it will only effect that codon

Question 18

What are the two stages of protein synthesis

Answer 18

Transcription- where the DNA sequence for one gene is copied into mRNA
Translation- where the mRNA joins with a ribosome and a corresponding tRNA molecule brings the specific amino acid the codon codes for.

Question 19

What is an intron

Answer 19

Non coding regions of DNA (they are removed (spliced) from mRNA molecules after transcription)

Question 20

What are exons

Answer 20

Regions of DNA that code for amino acids

Question 21

What are start/stop codons

Answer 21

Start codons allow the ribosome to attach
The stop codon is 3 bases that do not code for an amino acid, this causes the ribosome to detach and therefore ends translation

Question 22

Describe transcription

Answer 22

Process in which a complementary mRNA copy of one gene of the DNA is created in the nucleus
DNA helicase breaks the hydrogen bonds between the bases in the two strands of DNA
This causes the DNA helix to unwind and one strand acts as a template
Free mRNA nucleotides align opposite exposed complimentary DNA bases
RNA polymerase joins together the adjacent RNA nucleotides forming phosphodiester bonds to create a new mRNA polymer chain
The mRNA is modified and leaves the nucleus via nuclear pores

Question 23

Describe translation

Answer 23

The mRNA gets to the cytoplasm and attaches to the small subunit of the ribosome at the start codon
The tRNA molecule with the complementary anticodon to the start codon aligns opposite the mRNA (the ribosome can hold 2 tRNA molecules at one time)
The two amino acids that have been delivered by the tRNA molecule are joined together with a peptide bond which is catalysed by an enzyme using ATP
The ribosome will move along the mRNA molecule to the next codon and another complementary tRNA will attach to the next codon on the mRNA
This continues until the stop codon is reached and the ribosome detaches and ends translation
That polypeptide chain enters the golgi body for folding and modifications.