2.1.3 Nucleotides and Nucleic acids

Question 1

(c) the structure of ADP and ATP as phosphorylated nucleotides
Comprising a pentose sugar (ribose), a nitrogenous base (adenine) and inorganic phosphates.

Answer 1

• AMP has 1 inorganic phosphate
• ADP has 2 inorganic phosphates
• AMP has 3 inorganic phosphates

Question 2

(a) the structure of a nucleotide as the monomer from which nucleic acids are made
To include the differences between RNA and DNA nucleotides, the identification of the purines and pyrimidines and the type of pentose sugar.

Answer 2

Nucleotide: Monomer of nucleic acids (RNA or DNA) consisting of a five-carbon sugar, a phosphate group and a nitrogeneous base

purine = two rings e.g. adenine/guanine
pyramidine = one ring e.g. cytosine/thymine/uracil

Question 3

(d) (i) the structure of DNA (deoxyribonucleic acid)
To include how hydrogen bonding between complementary base pairs (A to T, G to C) on two antiparallel DNA polynucleotides leads to the formation of a DNA molecule, and how the twisting of DNA produces its ‘double-helix’ shape

Answer 3

• DNA (deoxyribonucleic acid) = polymer of nucleotides (deoxyribose sugar, nitrogeneous base + phosphate group)
• 2 DNA polynucleotides are antiparallel (running in opposite directions)
• Complementary pairing between nitrogeneous bases on each polynucleotide (sugar phosphate backbone)
  
  • purines pair with pyramidines
  • A pairs with T (2 H-bonds)
  • G pairs with C (3 H-bonds)
• Antiparallel chains twist to form double helix structure

Question 4

(e) semi-conservative DNA replication
To include the roles of the enzymes helicase and DNA polymerase, the importance of replication in conserving genetic information with accuracy and the occurrence of random, spontaneous mutations

Answer 4

• The double helix is unwinded and the two sugar phosphate backbones (DNA polynucleotides) are separated by DNA helicase
  
  • H-bonds = broken
  • leaving 2 single strands of DNA with exposed bases
• Free DNA nucleotides are hydrogen bonded to exposed nucleotides by complementary base pairing
• DNA Polymerase catalyses addition of new bases in the 5^’ to 3^’ direction
  
  • each single strand of DNA used a template
• phosphodiester bonds form between pentose sugar of one nucleotide and phosphate group of the next nucleotide
• two identical DNA molecules formed –> one old strand and one new strand

Mutations are random + spontaneous

• During replication, wrong nucleotide may be inserted (point mutation)
• could change genetic code, (advantageous/disadvantageous or have no effect because of degenerate code)

Question 5

(f) the nature of the genetic code
To include the triplet, non-overlapping, degenerate and universal nature of the code and how a gene determines the sequence of amino acids in a polypeptide (the primary structure of a protein)

Answer 5

• universal: in almost all living organisms the same triplet of DNA bases codes for the same amino acid
• triplet code: Each triplet (group of 3 bases) codes for an amino acid
• non overlapping: is read from a fixed point in groups of three bases. If a base added/deleted (random + spotantaneous) causes a frame shift mutation and every base changed after it
• degenerate: because most amino acids there is more than one triplet base code

Each gene (a sequence of DNA nucleotides) codes for 1 polypeptide (by transcription and translation)

Question 6

(g) transcription and translation of genes resulting in the synthesis of polypeptides
To include, the roles of RNA polymerase, messenger (m)RNA, transfer (t)RNA, ribosomal (r)RNA

Answer 6

Transcription:

• A gene unwinds and unzips
• Hydrogen bonds between complementary nucleotide bases break
• RNA polymerase catalyses the addition of temporary hydrogen bonds between RNA nucleotides and complementary DNA bases of the template strand
  
  • (A bonds to U = 2 H-bonds)
  • (G bonds to C = 3 H-bonds)
• length of RNA produced is a copy of the other DNA strand - the coding strand (except T’s are U’s)
• mRNA passes out of the nucleus through the nuclear envelope and attaches to a ribosome

Translation:

• Transfer RNA (tRNA - made in the nucleolus) carry specific amino acids and have a region of 3 nucleotides called the anticodon
• Anticodon binds to the complementary nucleotide bases (codon) of the mRNA strand by H-bonding
• Ribosome moves along length of mRNA
• 3 adjacent tRNA can bind to the mRNA at any one time
• Adjacent amino acids form peptide bonds