2.1.3: Nucleotides and nucleic acids Flashcards
Draw the structure of a nucleotide.
circle- phosphate group
pentagon- 5 carbon sugar
rectangle- 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 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
two-ring molecules
Name the pyrimidine bases and describe their
structure.
one-ring molecules
thymine C5H6N2O2 cytosine C4H5N3O uracil C4H4N2O2
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?
- Enzyme helicase unzips the double helix and the hydrogen bonds between bases break
- Free nucleotides line up with their complimentary bases and hydrogen bonds form
- DNA polymerase allows the free nucleotides to attach to their complimentary bases
- A complimentary strand has been formed for either template strand
- Any fragments in the double helix are joined by ligase
- There are now two identical DNA molecules formed – each with a daughter and parent strand
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 nucleotides are attracted to their complementary bases.
- RNA polymerase joins adjacent nucleotides to form phosphodiester bonds.
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 bases 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
ribose sugar
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.
