2.1.3 - Nucleotides and Nucleic Acids

Question 1

What is a nucleotide?

Answer 1

The monomer from which nucleic acids are made

Question 2

What is the structure of a nucleotide?

Answer 2

• Pentose sugar
• Nitrogen containing base
• Phosphate group

Question 3

What are purines?

Answer 3

2 carbon ring structures

Question 4

Which bases are purines?

Answer 4

• Adenine
• Guanine

Question 5

What are pyrimidines?

Answer 5

1 carbon ring structures

Question 6

Which bases are pyrimidines?

Answer 6

• Cytosine
• Thymine
• Uracil

Question 7

What is the structure of a DNA nucleotide?

Answer 7

• Deoxyribose sugar
• Phosphate group
• Organic base (A,T,C or G)

Question 8

What is the structure of a RNA nucleotide?

Answer 8

• Ribose sugar
• Phosphate group
• Organic base (A,U,C or G)

Question 9

What does a condensation reaction between two nucleotides form?

Answer 9

A phosphodiester bond to form a polynucleotide

Question 10

What does a hydrolysis reaction between two nucleotides do?

Answer 10

It breaks the phosphodiester bond between them

Question 11

What is the structure of ATP?

Answer 11

• Ribose sugar
• Adenine
• 3 phosphate groups

Question 12

What is the structure of ADP?

Answer 12

• Ribose sugar
• Adenine
• 2 phosphate groups

Question 13

What is the structure of DNA?

Answer 13

• Double stranded molecule that twists into a double helix
• Two sugar phosphate backbones held in place by pairs of complementary bases (A to T and C to G) joined by hydrogen bonds
• The two DNA strands are anti parallel running in opposite directions

Question 14

How is the structure of RNA different to DNA?

Answer 14

It is shorter and is a single stranded molecule

Question 15

How is DNA purified?

Answer 15

• Blend the DNA source from fruit or vegetable into a ‘soup’ and then add salt to remove DNA bound proteins and ice cold water
• Pass through a mesh and collect the liquid
• Add detergent to help disrupt the cell membrane and mix
• Add protease enzymes to hydrolyse the proteins
• Pour solution into a test tube and add ice cold ethanol at a 45° angle to cause the DNA to precipitate
• The DNA appears as white strands

Question 16

Why is DNA replication described as semi-conservative?

Answer 16

In replication one strand is conserved and one new strand is created

Question 17

How does semi-conservative DNA replication occur?

Answer 17

• DNA helicase unwinds the double helix and separates the two strands by breaking the hydrogen bonds between the complementary base pairs
• New DNA nucleotides align opposite to exposed complementary bases on the DNA template strand and hydrogen bonds form between them
• DNA polymerase catalyses the condensation reaction that joins the adjacent nucleotides together forming a phosphodiester bond and forming new strands
• The daughter DNA molecules each rewind into a double helix

Question 18

Why is replication important?

Answer 18

It conserves genetic information with accuracy but copying errors can occur randomly and spontaneously resulting in a change to the DNA base sequence known as a mutation

Question 19

What is the genetic code?

Answer 19

It is carried as a sequence of three DNA bases called a triplet or codon which code for a specific amino acid or code for STOP during transcription

Question 20

What are the 3 key features of the genetic code?

Answer 20

• Universal - same triplet of bases code for the same amino acid in all organisms
• Non-overlapping - each base in a gene is only part of one triplet of bases that codes for one amino acid and each codon is read as a discrete unit
• Degenerate - amino acids are coded for by more than one triplet code

Question 21

What is a gene?

Answer 21

A sequence of triplets specifying the order of amino acids of a polypeptide or protein

Question 22

What is protein synthesis?

Answer 22

The mechanism by which a DNA template is transcribed into a messenger RNA (mRNA) molecule in the nucleus. The mRNA is then translated into an amino acid sequence in association with transfer RNA (tRNA) on ribosomes in the cytoplasm

Question 23

What are introns?

Answer 23

Sequences of bases in a gene that do not code for amino acids and therefore polypeptide chain. These get spliced out of mRNA molecules after transcription

Question 24

What are exons?

Answer 24

Sequences of bases in a gene that code for a sequence of amino acids

Question 25

What is the process of transcription?

Answer 25

• DNA helicase breaks hydrogen bonds between bases causing DNA to unzip
• Free mRNA nucleotides align opposite exposed complementary DNA bases
• The enzyme RNA polymerase joins together the adjacent RNA nucleotides forming phosphodiester bonds to create a new mRNA polymer chain
• Once one gene is copied the mRNA is modified and then leaves the nucleus through the nuclear envelope pores

Question 26

What is the process of translation?

Answer 26

• mRNA moves from the nucleus through a nuclear pore to the cytoplasm and the start codon attaches to a ribosome
• A tRNA with a complementary anticodon carrying a specific amino acid moves to the ribosome and pairs with the first mRNA codon
• The ribosome moves along the mRNA to the next codon and again pairs up with a complementary tRNA to bring the two amino acid carrying tRNAs together
• Energy released from ATP is used to form a peptide bond between the amino acids
• The ribosome moves to the third mRNA codon releasing the first tRNA and pairing up a third
• When the ribosome reaches a stop codon the polypeptide is complete and the mRNA and tRNAs are released from the ribosome