Nucleotides And Nucleic Acids

Question 1

Structure of a nucleotide

Answer 1

• pentose sugar (deoxyribose/ribose)
• phosphate grp
• nitrogenous base=adenine, cytosine, guanine, thymine/uracil
• joined by condensation reaction
• overall-ive (due to phosphate)

Question 2

Purine vs pyrimidine bases

Answer 2

• purine=Pure As Gold=gold hoop earrings
  Adenine, guanine (double ring structure)
• pyrimidine= CUT pie
  Cytosine, uracil, thymine (single ring structure)

Question 3

How do polynucleotides form?

Answer 3

• condensation reaction=phosphodiester bonds

Question 4

Structure of ADP+ATP

Answer 4

• phosphorylated nucleotides
• structure
  Ribose+adenine=adenosine & 2/3 phosphate grps

Question 5

ATP:properties+function

Answer 5

• properties
  Universal energy currency
  Small=lots of it can be stored+move quickly
  Water soluble as most reactions occur in cytoplasm+ATP has to be next to them
  Bonds bw phosphate grips=unstable, low Ea=easily broken
  Releases energy in small quantities=more stable energy lvls
  Easily regenerated=readily available
  Phosphate can phosphorylate other compounds=makes them more reactive
• function
  ATP+H2O —> ADP+Pi
  catalysed by ATP hydrolase and releases energy for use in cells
  ADP+Pi —> ATP+H2O
  Catalysed by ATP synthase & requires energy and traps chemical energy in the bond

Question 6

Structure of DNA

Answer 6

• discovered by Watson and Crick in 1953
• Sugar-phosphate backbone=protects coding bases on the inside of the helix+run antiparallel to each other one goes 5’ to 3’ and vice versa
• Double stranded=allows strands to act as templates in DNA replication
• Large molecule=stores lots of info
• Double helix=molecule twists=compact molecule
• Complementary base pairing=allows accurate DNA replication (A+T form 2 H-bonds, C+G form 3 H-bonds)
• Weak hydrogen bonds=allows strands to split in DNA replication

Question 7

DNA replication (semi-conservative)

Answer 7

• DNA helicase breaks H-bonds bw complementary bases=unwinds double helix+splits strands
• free activated nucleotides=attracted to their complementary base pairs
• DNA polymerase catalyses the formation of phosphodiester bonds bw nucleotides via condensation reactions, in the 5’ to 3’ direction
• 2 identical copies of DNA are made

Question 8

Meselson-Stahl experiment

Answer 8

• proves semi-conservative replication
• Bacteria were grown in a medium containing , so all their DNA is ‘heavy’=N-15
• The bacteria were transferred to a medium with for one round of replication, so the lighter nitrogen was incorporated into any new DNA strands they made.
• The DNA was extracted and centrifuged.
• Steps 2-3 were repeated for another round of replication.
• The distribution of heavy and light DNA was analysed to track how the DNA was replicating:
• The heavier bands sink lower in the test tube.
• The intermediate bands, made of DNA with one heavy strand and one light strand, are in the middle of the test tube.
• The lighter bands are higher up in the test tube.

Question 9

The nature of genetic code

Answer 9

• universal=same 4 bases used+same triplet codes for the same amino acid
• non-overlapping=each base is only read once
• degenerate=amino acids are coded by one more than one triplet=random mutations x affect the amino acid being coded

Question 10

Protein synthesis: transcription

Answer 10

-transcription
RNA polymerase binds to DNA
H-bonds bw DNA bases break+strands split
- antisense strand acts as the template for mRNA synthesis
Free RNA nucleotides align with the DNA template through complementary base pairing (A+U) (C+G)
RNA polymerase catalyses the formation of phosphodiester bonds bw RNA nucleotides.
A complementary mRNA strand is formed, carrying the same base sequence as the DNA sense strand.
The process ends when RNA polymerase reaches a stop codon, detaches from DNA and terminates transcription.
mRNA is released, detaches from DNA, and DNA rewinds into its double helix structure.

Question 11

Protein synthesis:translation

Answer 11

• ribosome attaches to the mRNA strand at a start codon
• tRNA molecule, carrying a specific amino acid and with an anticodon=complementary to start codon, binds to the mRNA.
• second tRNA molecule with an anticodon complementary to the next mRNA codon, and also carrying a specific amino acid, attaches to mRNA.
• amino acids carried by the first two tRNA molecules are linked together via a peptide bond using ATP
• first tRNA molecule detaches from mRNA and is free to collect another amino acid for future use.
• ribosome moves along mRNA, allowing another tRNA molecule, which carries the next amino acid, to bind to the next codon on mRNA.
  The process from step 4 to 6 is repeated, which elongates the polypeptide chain.
  At any point during this process, two tRNA molecules can be attached to the ribosome.
  The sequence continues until the ribosome reaches a stop codon on mRNA.
  The completed polypeptide chain detaches from the ribosome.