alevel biology ocr nucleic acids and nucleotides

name the pentose sugars in DNA and 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

name the pyrimidine bases and describe their structure

thymine: C5H6N2O2 cytosine: C4H5N3O uracil: C4H4N2O2

name the complimentary base pairs in DNA and RNA

DNA: 2 H-bonds between Adenine (A) and thymine (T)
RNA: 2H-bonds between Adenine (A) and thymine (T)
both have 3 H-bonds between Guanine (G) and Cytosine (C)

Why is DNA replication called semi-conservative?

Strands from original DNA molecule act as templates.
New DNA molecule contains 1 old strand & 1 new strand

Explain the role of DNA helicase in semiconservative replication.

breaks H-bonds between base pairs to form two single strands, each of which can act as a template

how is a new strand formed during semi-conservative replication

1. free nucleotides from nuclear sap attach to exposed bases by complimentary base pairing
2. DNA polymerase joins adjacent nucleotides on new strand in a 5' to 3' direction via condensation reactions to form phosphodiester bonds
3. 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 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 and occurs in nucleus

outline the process of transcription

1. RNA polymerase binds to promoter region on a gene.
2. Section of DNA uncoils into 2 strands with exposed bases. Antisense strand acts as template.
3. Free nucleotides are attracted to their complementary bases.
4. 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

Outline the process of translation

1. Ribosome moves along mRNA until 'start' codon.
2. tRNA anticodon attaches to complementary bases on mRNA.
3. Condensation reactions between amino acids on tRNA form peptide bonds. Requires energy from ATP hydrolysis.
4. Process continues to form polypeptide chain until 'stop' codon is reached.

describe the structre of ATP and ADP

nucleotide derivative of adenine
ATP has 3 inorganic phosphate groups
ADP has 2

what is a mutation

change in DNA sequence