Nucleotides & nucleic acids

Question 1

What are the two types of nucleic acids? And what do they stand for?

Answer 1

• DNA (deoxyribonucleic acid)
• RNA (ribonucleic acid)

Question 2

What are the elements found in nucleic acids?

Answer 2

Carbon, hydrogen, oxygen, nitrogen and phosphorus

Question 3

What is a purine? And provide examples

Answer 3

• The two ringed base in a nucleotide
• e.g. adenine and guanine

Question 4

What is a pyrimidine?

Answer 4

• The single ringed base in an nucleotide
• e.g. thymine, uracil and cytosine

Question 5

Describe the structure of a DNA molecule?

Answer 5

• 2 polynucleotide chains/strands hydrogen bond together to make a DNA double helix molecule => deoxyribose pentose sugar and phosphate group make up the outside backbone
• Bases hydrogen bond in the centre
• These bases H bond together because they are complementary to eachother
• Purines must pair with pyrimidine so the width of the helix is consistent
• The two strands are antiparallel => they run in opposite directions i.e. 5’ to 3’ and 3’ to 5’
• The two polynucleotide strands twist to form a double helix

Question 6

How many hydrogen bonds are between A and T, and G and C?

Answer 6

• Between A and T are 2 H bonds
• Between G and C are 3 H bonds

Question 7

What is a nucleotide up of?

Answer 7

• A pentose monosaccharide (sugar), containing five carbon atoms
• A phosphate group, an inorganic molecule that is acidic and negatively charged
• A nitrogenous base

Question 8

Draw deoxyribose sugar. And give the chemical formula.

Answer 8

Answer on revision card

Question 9

Draw ribose sugar. And give the chemical formula.

Answer 9

Answer on revision card

Question 10

What does ATP stand for?

Answer 10

Adenosine triphosphate

Question 11

What are the three main types of activities that the cell needs energy for?

Answer 11

• Synthesis
• Transport
• Movement

Question 12

Draw the structure of ATP.

Answer 12

Answer on revision card

Question 13

What is the formula for the hydrolysis reaction of ATP?

Answer 13

Answer on revision card

Question 14

What are the properties of ATP?

Answer 14

• Small => moves easily into, out of and within cells
• Water soluble/hydrophilic => energy-requiring processes happen 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 => quantities are suitable to most cellular needs, so that energy is not wasted as heat
• Easily regenerated => is reformed from ADP and Pi in one reaction

Question 15

What are histones?

Answer 15

Positively charged proteins that form a complex with DNA called chromatin

Question 16

What is the process of DNA replication?

Answer 16

• Gyrase enzyme relaxes the super coils of the DNA helix
• Helicase enzyme unwinds the helix and seperates the two strands by breaking the hydrogen bonds between the two strands: the DNA molecule is ‘unzipped’
• Both strands will act as a template strand. These are called the parent strands. Complementary base pairing by the formation of hydrogen bonds will occur between the bases of each parent strand and the bases of free nucleotides starting the formation of two new daughter strands
• Each free nucleotide is added as a triphosphate and loses two phosphates molecules in the condensation reaction when added to the growing daughter strand
• DNA polymerase enzyme catalyses the formation of a phosphodiester bond between the deoxyribose and phosphate of two adjacent nucleotides on the daughter strand through a condensation reaction enabling the daughter strand to grow/polymerise
• Nucleotides are addedin the 5’ to 3’ direction. The leading strand is formed continously. The other strand, the lagging strand is formed discontinuously using okazaki fragments which are joined together by the enzyme ligase
• Mutations (a change from the correct base sequence of DNA) may occur by mistakes made in DNA replication
• Enzyme proofread and correct these mutations in the G2 phase of interphase following DNA replication in S phase

Question 17

What is semi-conservative replication?

Answer 17

• Each daughter molecule has conserved one of the original parent strands
• The process by which both strands of a DNA double helix act as a template for the synthesis of a new complementary strand resulting in 2 new DNA molecules each of which has one parent (old) and one daughter (new) strand

Question 18

What is the genetic code?

Answer 18

The base or nucleotide sequence of the DNA which codes for the aa sequence of proteins

Question 19

What is a codon?

Answer 19

3 adjacent bases/nucleotides that code for an amino acids

Question 20

What is a genome?

Answer 20

All the DNA that is present in an organism

Question 21

What are the properties of the genetic code?

Answer 21

• It is degenerate => some amino acids have more than 1 possible condon, coding for them
• It is non-overlapping => each codon is read separately from the codon before and after it. No base is shared between adjacent codons
• It is universal => the same specific codons code for the same amino acids in all living organisms

Question 22

What is a mutation?

Answer 22

A change in the base sequence of the DNA

Question 23

What are the effects of mutations?

Answer 23

• Most mutations are harmless as they occur in non-coding DNA. Also, the degenerate nature of the genetic code means that a change in the base sequence of a codon may still code for the same as. These mutations are known as silent mutations
• Some are deleterious as they occur in a codon of a gene so that the base sequence is changed and the wrong aa is coded for in the protein. Therefore the primary structure of the protein is changed and so the final folding (tertiary structure) and its function is lost. In the case of an enzyme, the shape of the active site is lost
• Very occasionally, mutations are advantageous and bring about evolution by natural selection

Question 24

What is the process of transcription in protein synthesis?

Answer 24

• The sense strand (5’ to 3’) of DNA contains the code (it is the coding strand) for the protein to be synthesised
• The antisense strand (3’ to 5’) acts as the template strand so that the RNA strand formed is a copy of the sense strand
• The DNA helix is unwound at the start of the gene
• The hydrogen bonds between the 2 strands are broken by helicase
• Free complementary ribonucleotides form H bonds with the template strand
• A condensation reaction is carried out by RNA polymerase
• Phosphodiester bonds form the sugar phosphate backbone of the growing RNA chain
• The coding strand is not transcribed
• When transcription of the gene is finished, mRNA leaves the nucleus through the nuclear pores

Question 25

What is the process of translation in protein synthesis?

Answer 25

• The mRNA binds to the small subunit of the ribosome at its start codon (always AUG)
• The first tRNA with the complementary anticodon, moves into the ribosome and hydrogen bonds to the mRNA start codon. This tRNA carries the amino acid methionine
• The second tRNA eneterds the ribosome (carrying aa2) and its anticodon hydrogen bonds with the codon on the mRNA
• A peptide bond is formed between aa1 and aa2 by a condensation reaction catalysed by the enzyme peptidyl transferase
• The ribosome then moves along to the next condon on the mRNA and releases the first tRNA
• The process is repeated until the ribosome reaches the end of the mRNA and a stop codon is reached
• Translation is terminated and the ribosome seperates from the mRNA and the primary structure of the polypetide has been made

Question 26

What is tRNA?

Answer 26

• A form of RNA that is composed of a strand of RNA folded in a way that three bases, called the anticodon are at one end of the molecule
• The anticodon has the complementary sequence to the condon on mRNA. This hydrogen bonds with the codon on the mRNA inside the ribosome
• The tRNA molecules carry an amino acid corresponding to that codon