3.8 - 3.11 Nucleic Acids

Question 1

What are nucleic acids?

Answer 1

Large polymers formed from many nucleotides (the monomers) linked together in a chain.

• contain the elements: carbon, hydrogen, oxygen, nitrogen and phosphorus.
• two types: DNA and RNA.

Question 2

What is the role of DNA and RNA?

Answer 2

Storage and transfer of genetic information and the synthesis of polypeptides (proteins). They are the basis for hereditary.

Question 3

What is a nucleotide made up of?

Answer 3

Three components:

1. Pentose (5 carbon atoms) monosaccharide.
2. Phosphate group. (Acidic and negatively charged inorganic molecule)
3. Nitrogenous base. (complex organic molecule containing one or two carbon rings in its structure as well as nitrogen).

Question 4

Explain the formation of a polynucleotide.

Answer 4

Nucleotides are linked together by condensation reactions.

• the phosphate group at the 5th carbon of the pentose sugar of one nucleotide forms a covalent bond with the hydroxyl (OH) group at the 3rd carbon of the pentose sugar of an adjacent nucleotide.
• these bonds = phosphodiester bonds.
• this forms a long, strong sugar phosphate backbone with a base attached to each sugar.

Question 5

How can phosphodiester bonds be broken?

Answer 5

By hydrolysis, releasing the individual nucleotides.

Question 6

Explain the structure of DNA.

Answer 6

The sugar and phosphate group is always the same.
Sugar = deoxyribose (has one fewer oxygen atom than ribose).

Base can vary- each nucleotide has one of four bases:
-thymine (T)
-cytosine (C)
- adenine (A)
- guanine (G)
therefore there are 4 different DNA nucleotides

Question 7

What are the two types of bases? (DNA)

Answer 7

Pyrimidines- smaller bases, which contain single ring structures. Thymine and cytosine.

Purines- larger bases, contain double ring structures. Adenine and guanine.

Question 8

Explain the DNA double helix.

Answer 8

DNA is made up of two strands of polynucleotides coiled into a helix.

• the two strands are held together by hydrogen bonds between the bases.
• each strand has a phosphate group (5’) at one end and a hydroxyl group (3’) at the other.
• the two parallel strands are arranged so they run in opposite directions (anti parallel).

Question 9

Explain complimentary base pairing.

Answer 9

Adenine and thymine are able to form 2 hydrogen bonds and always join with each other.
Cytosine and guanine form 3 hydrogen bonds and therefore only bind to each other.

• these rules mean a small pyrimidine base always bings to a larger purine base and this maintains a constant distance between the DNA backbones.
• also means that DNA always gas equal amounts of A and T and equal amounts of C and G.

Question 10

What is RNA?

Answer 10

Ribonucleic acid plays an essential role in the transfer of genetic information from DNA to proteins that make up the enzymes and tissue in body.
-DNA is too large to leave the nucleus so the section of the DNA molecule corresponding to a single gene is transcribed to mRNA which is much shorter.

Question 11

How are the nucleotides of RNA different to those of DNA?

Answer 11

In RNA pentose sugar= ribose.
-thymine base is replaced by uracil (U)
-uracil is a pyrimidine that forms 2 hydrogen bonds with adenine.
Therefore base pairing rules still apply.

After protein synthesis the RNA molecules are degraded in the cytoplasm. The phosphodiester bonds are hydrolysed and the RNA nucleotides are released+reused.

Question 12

Explain semi-conservative DNA replication?

Answer 12

• for dna to replicate, the double helix has to unwind and then separate into 2 strands, so the hydrogen bonds holding the complimentary bases together must be broken.
• free dna nucleotides will then pair with their complimentary bases, which have been exposed as the strands separate. Hydrogen bonds are formed between them.
• finally, the new nucleotides join to their adjacent nucleotides with phosphodiester bonds.
• 2 new DNA molecules produced. Each consists of one old dna strand and one new strand, hence the name semi-conservative (half same).

Question 13

What are the roles of enzymes in DNA replication?

Answer 13

DNA helicase - carries out the unwinding and separating of the two strands of the DNA helix before replication can occur.
Travels along the dna backbone, catalysing reactions that break the hydrogen bonds between complimentary base pairs.

DNA polymerase- catalyses the formation of phosphodiester bonds between the free nucleotides and the newly exposed bases on the template strands.

Question 14

Explain continuous and discontinuous replication. (DNA)

Answer 14

DNA polymerase can only bind to the 3’ (OH) end, so travels in the direction of 3’ to 5’. (The other strand travels 5’ to 3’ as they are anti parallel)
As DNA only unwinds in one direction, dna polymerase must replicate each of the template strands in opposite directions.
-The strand that unzips from the 3’ end can be continuously replicated as the strands unzip. It is called the leading strand and is said to undergo continuous replication.
- the other strand unzips from the 5’ end, so DNA polymerase has to wait until a section of the strand has been unzipped, then work back along the strand. This results in dna being produced in sections called okasaki fragments, which then have to be joined. Dna ligase catalyses the joining of these fragments.
This is the lagging strand which undergoes discontinuous replication.

Question 15

Explain a mutation.

Answer 15

Can occur when sequences of bases are not matched exactly.

These errors occur randomly and spontaneously.

Question 16

Explain the genetic code.

Answer 16

The sequences of bases un DNA are the instructions for the sequences of amino acids in the production of proteins.
The genetic code is universal- all organisms use this same code, although the sequences of bases coding for each individual protein will be different.

Question 17

What is triplet code and a codon?

Answer 17

The code in the base sequences is a simple triplet code.
It is a sequence of three bases, called a codon.
Each codon codes for an amino acid

There are 4 bases meaning there are 64 different base triplets/codons.

Question 18

What is a gene?

Answer 18

A section of DNA that contains the complete sequences of bases (codons) to code for an entire protein is called a gene.

Question 19

Explain how the genetic code is degenerate.

Answer 19

The same amino acid can be coded for by more than one codon.

There are 4 bases meaning there are 64 different base triplets/codons. Includes:

• 1 codon which acts as the start codon when it comes at the beginning of a gene. It signals the start of a sequence that codes for a protein.
• if it is in the middle, it codes for the amino acid methionine.
• 3 stop codons that don’t code for any amino acids and signal the end of a sequence.

Question 20

What is meant by the genetic code being non-overlapping?

Answer 20

No single base can take part in the formation of more than one codon.

The 1 codon at the start ensures that the genetic code is non-overlapping.

Question 21

What is transcription?

Answer 21

The process of copying sections of DNA base sequences to produce smaller molecules of mRNA.
This happens because DNA molecules are too large to leave the nucleus.

Question 22

Explain the stages of transcription.

Answer 22

1. Section of DNA containing gene unwinds and unzips by DNA helicase, beginning at a start codon. Involves breaking of hydrogen bonds between bases.
2. The antisense strand acts as a template strand, so that the complimentaryRNA strand formed carries the same base sequence as the sense strand.
3. Free RNA nucleotides will base pair with complimentary bases exposed on the anti sense strand when the DNA unzips.
4. Phosphodiester bonds are formed between the RNA nucleotides by enzyme RNA polymerase.
5. Transcription stops at the end of the gene and mRNA is formed. It detaches from the DNA template and leaves the nucleus through a nuclear pore.
6. DNA double helix reforms and mRNA travels to a ribosome.

Question 23

What is the sense and antisense strand? (Transcription)

Answer 23

Only one of the two strands of DNA contains the code for the protein to be synthesised.

• This is the sense strand- it runs from 5’ to 3’.
• The other strand is the antisense strand- runs 3’ to 5’. It is a complimentary copy of the sense strand and does not code for a protein but instead acts as a template during transcription.

Question 24

What is translation?

Answer 24

The process by which the complimentary code carried by mRNA is decoded by tRNA into a sequence of amino acids.

Question 25

What is ribosomal RNA?

Answer 25

• the subunits that make up a ribosome.
• maintains the structural stability of the protein synthesis sequence.
• plays a biochemical role in catalysing the reaction.

Question 26

What is transfer RNA?

Answer 26

A form of RNA that carries an amino acid specific to its anticodon to the correct position along mRNA in translation.
-composed of a strand of RNA folded in a way that three bases (anticodon), are at one end of the molecule.
The tRNA molecules carry an amino acid corresponding to that codon.
-when the tRNA anticodons bind to the complimentary codons along the mRNA, the amino acids are brought together in the correct sequence to form the primary structure of the protein coded for by the mRNA.
-the amino acids are added one at a time and the polypeptide chain grows as this happens.
-ribosomes act as the binding site for mRNA and tRNA and catalyse the assembly of the protein.

Question 27

Explain the stages of translation.

Answer 27

1. mRNA binds to the small subunit of the ribosome at its start codon (AUG)
2. A tRNA with the complimentary anticodon (UAC) binds to the mrna start codon. (This trna carries the amino acid methionine)
3. Another tRNA

Question 28

What are the main activities cells require energy for?

Answer 28

1. Synthesis- eg. large molecules like proteins.
2. Transport- eg. Pumping molecules/ ions across cell membranes by active transport.
3. Movement- eg. Muscle contraction.

Question 29

What is ATP (and structure)?

Answer 29

Inside cells, molecules of adenosine triphosphate supply energy.
- composed of a nitrogenous base, a pentose sugar and three phosphate groups.
-Base= adenine.
-Sugar= ribose.
Known as universal energy currency because it is used for energy transfer in all cells of all living things.

Question 30

How does ATP release energy?

Answer 30

• only a small amount of energy is needed to break the weak bond holding the last phosphate group. However, a large amount of energy is released later when the phosphate undergoes other reactions. Overall more energy is released than used.
  -hydrolysis reaction:
  ATP + H20 —> ADP + P1 + energy

Question 31

Is ATP a good long term store?

Answer 31

ATP isn’t a good long-term store due to the instability of the phosphate bonds.

• the energy released in the breakdown of fats/carbohydrates creates ATP by reattaching a phosphate group to an ADP molecule. This is called phosphorylation and is a condensation reaction.
• due to instability, cells do not store large amounts of it, however ATP can be rapidly reformed by the phosphorylation of ADP so is therefore a good immediate energy

Question 32

What are the properties of ATP?

Answer 32

• small: moves easily in/out of cells.
• water soluble: energy-requiring processes happens in aqueous environments.
• contains bonds between phosphates with intermediate energy: large enough to be useful for cellular reactions but not so large that energy is wasted as heat.
• releases energy in small quantities: suitable to cellular needs.
• easily regenerated: can be recharged with energy.