Chapter 3: Nucleic Acids

Question 1

Nucleotide structure

Answer 1

Many nucleotide monomers link to form a nucleic acid polymer (contain C, H, O, N and P)
Made of
- a pentose monosaccharide sugar with 5 C atoms (ribose in RNA, Deoxyribose in DNA)
- a phosphate group (PO4-3) inorganic, negatively charged
- a nitrogenous base (complex organic molecule containing N and 1/2 C rings)

Question 2

Polynucleotides

Answer 2

Synthesis:
Condensation reactions between adjacent nucleotides where the phosphate group at 5’ forms a phosphodiester bond with the OH group at 3’ and water molecules are released
Breakdown:
Hydrolysis reactions where water is added to break the phosphodiester bonds and individual nucleotides are released

Question 3

Nitrogenous bases

Answer 3

Pyrimidines:
Group of smaller nitrogenous bases, containing single carbon ring structures
E.g. C and T

Purines:
Group of larger nitrogenous bases, containing double carbon ring structures
E.g. A and G

Question 4

ATP structure

Answer 4

Energy transfer in cells for synthesis, movement and transport
Made of
- adenine (a nitrogenous base, purine)
- a pentose monosaccharide sugar with 5 C atoms
- three phosphate groups (PO4-3)

Question 5

Energy release from ATP

Answer 5

Hydrolysis reaction:
ATP + H2O → ADP + Pi + energy
ATP hydrolysed to ADP and a phosphate ion to release energy
Phosphate bonds in ATP are unstable so cannot store energy long-term
Good immediate energy store
Created by energy released during cellular respiration when breaking down long-term energy stores (e.g. fats, carbohydrates)

Question 6

ADP

Answer 6

Adenosine diphosphate
Used to make ATP in phosphorylation process
- reattach phosphate ion to ADP molecule in a condensation reaction
Condensation and hydrolysation is reversible - recycling system

Photophosphorylation:
- in the chlorophyll during photosynthesis
- uses light
Oxidative phosphorylation:
- in the mitochondria during the electron transport chain
- part of respiration
Substrate-level phosphorylation:
- phosphate ions are transferred from donor molecules to ADP

Question 7

Complementary base pairing

Answer 7

Nitrogenous bases bind specially where one purine binds to one pyrimidine
Adenine forms 2 hydrogen bonds with thymine
Cytosine forms 3 hydrogen bonds with guanine
There is always equal amounts of A=T and C=G

Question 8

DNA Double Helix

Answer 8

Two polynucleotide strands held together by H bonds between bases
Coil into a double helix shape
Each strand has a phosphate group at the 5’ end and an OH group at the 3’ end
Antiparallel strands run in opposite directions

Question 9

Semi conservative replication

Answer 9

In DNA replication the double helix separates and each strand serves as a template spike a new double-stranded DNA molecule

Each new molecule consists of one original DNA strand and one new strand

Question 10

DNA Helicase

Answer 10

Enzyme that separates the 2 strands in the double helix
Travels along the phosphate sugar backbone and catalyses reactions that break down the hydrogen bonds between nitrogenous bases

Question 11

DNA Polymerase

Answer 11

Enzyme that joins free nucleotides to their complementary bases
Travels along the template strand from the 3’ end to the 5’ end and catalyses the formation of phosphodiester bonds between nucleotides
Leading strand unzipped from the 3’ can undergo continuous replication
Lagging strand unzipped from the 5’ has to wait until a section has been unzipped and then work backwards along the strand - undergoes discontinuous replication
- DNA is produced in sections called Okazaki fragments that need to be joined

Question 12

Mutation

Answer 12

Sequence of bases in DNA important for genetic code - DNA replication ensures that 2 daughter cells are genetically identical to parent cell

Errors in replication when sequence of bases are not exactly matched can lead to a change in sequence of bases - mutation
Occurs randomly and spontaneously

Question 13

The triplet code

Answer 13

A sequence of 3 bases - codon
Each codon codes for an amino acid
The sequence of DNA is read in threes, and the section of DNA that codes for an entire protein is a gene
Genetic code is universal as all living organisms use the same code

Question 14

Degenerate code

Answer 14

64 different base codons possible (4 x 4 x 4)
Many amino acids can be specified by more than one codon - more codons than amino acids (only 29 regularly occurring amino acids)
Each codon only species one amino acid/ stop signal so there is no uncertainty

If there is a mistake in 1 base then the overall genetic code is unlikely to be affected, correct amino acid is still produced - avoids mutations

Question 15

Non-overlapping

Answer 15

Genetic code had a start codon (AUG) to signal the start pf the sequence - ensures codons are read in the correct reading frame (read from base 1, no overlap)

Non-coding DNA acts as a buffer zone between the coding bases

Three stop codons to signal the end of a sequence

Question 16

RNA polymerase

Answer 16

Enzyme makes a transcript of the DNA sequence
Catalyses the formation of phosphodiester bonds between the free nucleotides and the complementary bases exposed on the antisense strand to form mRNA

Question 17

mRNA

Answer 17

Carries genetic information from the protein-coding gene in the DNA in the nucleus, to the ribosomes - site of protein synthesis
DNA is too large to leave the nucleus but mRNA is a single strand short enough to leave the nuclear envelope

Question 18

Transcription

Answer 18

Antisense strand is used as the template as the mRNA formed will then be identical to the sense strand (from the 5’ to 3’ end) that codes for the required protein

When the DNA is unzipped, the free nucleotides align along the exposed bases on the antisense strand in complementary base pairing. RNA polymerase joins the RNA nucleotides together by reforming the phosphodiester bonds between them to make mRNA

Question 19

Translation

Answer 19

1) mRNA associates with a site on the small subunit of a ribosome at its start codon (AUG)
2) a tRNA with the complementary anticodon (UAC) bind to the mRNA start codon and carries the specified amino acid (methionine)
3) another tRNA with the next complementary anticodon carries the specified amino acid and bonds to the codon on the mRNA. Max of 2 tRNAs can be bound at the same time.
4) ribosome moves along the mRNAk releasing the first tRNA. The second becomes the first.
5) Process repeats until stop codon is reached and polypeptide is released.

Question 20

rRNA

Answer 20

Major component in ribosomes, important in maintaining the structural stability of protein synthesis sequence
helps mRNA bind to the right spot on the small subunit of the ribosome, so that the base sequence can be read out
Holds mRNA in position while it is translated into a sequence of amino acids

Question 21

tRNA

Answer 21

Necessary during translation by acting as carriers to bring amino acids to the ribosome
Ensures the amino acid that is added to the chain is corresponding to the codon specified by the mRNA
Consists of a single strand of RNA folded so complementary segments stick together, forming double-stranded regions
3 bases at one end of the molecule - anticodon
Bind to complementary codon on MRNA to form the primary structure of the protein.