2.1.3 - Nucleotides and Nucleic Acids Flashcards
Describe the structure of a nucleotide.
Pentose sugar is bound to a phosphate group and a nitrogenous base
Name the pentose sugars in DNA & RNA
DNA: deoxyribose
RNA: ribose
Describe how polynucleotide strands are formed and broken down
Condensation reactions between nucleotides form strong phosphodiester bonds (sugar-phosphate backbone). Hydrolysis reactions use a molecule
of water to break these bonds.
Enzymes catalyse these reactions.
Describe how polynucleotide strands are formed and broken down.
Condensation reactions between nucleotides form strong phosphodiester bonds (sugar-phosphate backbone). Hydrolysis reactions use a molecule of water to break these bonds. Enzymes catalyse these reactions.
Describe the structure of DNA.
Molecule twists to form double helix of 2 deoxyribose polynucleotide strands (so there are 2 sugar-phosphate backbones). H-bonds form between complementary base
pairs (AT & GC) on strands that run antiparallel.
Name the purine bases and describe their structure.
adenine C5H5N5
guanine C5H5N5O
Both are two ring molecules
**Trick to remember:
Purine = pure
AnGels are pure and A and G refer to the bases
Angels have TWO wings = two ring molecules
Name the pyrimidine bases and describe
their structure.
cytosine C4H5N3O
uracil C4H4N2O
thymine C5H6N2O2
These are all one-ring molecules
**Trick to remember:
You can CUT a PYRAMID from the single pointy top
CUT = Cytosine, Uracil, Thymine
Pyramid = pyrimidine
SINGLE pointy top = ONE-ring molecule
Name the complementary base pairs in DNA and RNA.
DNA: 2 H-bonds between adenine (A) + thymine (T)
RNA: 2 H-bonds between adenine (A) + uracil (U)
Both have 3 H-bonds between guanine (G) + cytosine (C)
Why is DNA replication described as semiconservative?
Strands from original DNA molecule act as
templates. New DNA molecule contains 1 old strand & 1
new strand (specific base pairing enables genetic material to be conserved accurately).
Explain the role of DNA helicase in semiconservative
replication.
Breaks H-bonds between base pairs to form 2 single strands, each of which can act as a template.
How is a new strand formed during semiconservative
replication?
- Free nucleotides from nuclear sap attach to exposed bases by complementary base pairing.
- DNA polymerase joins adjacent nucleotides on new strand in a 5’ → 3’ direction via condensation reactions to form phosphodiester bonds.
- H-bonds reform.
Identify features of the genetic code.
● Non-overlapping: each triplet is only read once.
● Degenerate: more than one triplet codes for the same amino acid (64 possible triplets for 20 amino acids).
● Universal: same bases and sequences used by all species.
How does a gene determine the sequence of amino
acids in a protein?
Consists of base triplets that code for a
specific amino acids.
Describe how DNA can be purified by precipitation.
Add ethanol & a salt to aqueous solution.
Nucleic acids precipitate out of solution.
Centrifuge to obtain pellet of nucleic acid.
Wash pellet with ethanol & centrifuge again.
What does transcription produce and where does it
occur?
produces mRNA
occurs in nucleus
Outline the process of transcription.
- RNA polymerase binds to promoter region on a gene.
- Section of DNA uncoils into 2 strands with exposed bases. Antisense strand acts as template.
- Free RNA nucleotides are attracted to their complementary bases.
- RNA polymerase joins adjacent nucleotides to form phosphodiester bonds to produce mRNA
What happens after a strand of mRNA is transcribed?
● RNA polymerase detaches at terminator region.
● H-bonds reform & DNA rewinds.
● splicing removes introns from pre-mRNA in eukaryotic cells.
● mRNA moves out of nucleus via nuclear pore & attaches to ribosome
What does translation produce and where does it occur?
Produces proteins
Occurs in cytoplasm on ribosomes (which are made of protein + rRNA)
Outline the process of translation.
- Ribosome moves along mRNA until ‘start’ codon.
- tRNA anticodon attaches to complementary codon on mRNA.
- Condensation reactions between amino acids on tRNA form peptide bonds. Requires energy from ATP hydrolysis.
- Process continues to form polypeptide chain until ‘stop’ codon is reached.
Describe the structure of adenosine triphosphate
(ATP) and adenosine diphosphate (ADP).
nucleotide derivative of adenine
ATP has 3 inorganic phosphate groups
ADP has 2
What is a mutation?
An alteration to the DNA base sequence.
Mutations often arise spontaneously during DNA replication
