1.5 Nucleic acids

Question 1

The components of a DNA nucleotide are…

Answer 1

deoxyribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or thymine.

Question 2

The components of an RNA nucleotide are…

Answer 2

ribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or uracil.

Question 3

Describe the structure of DNA.

Answer 3

1. Polymer of nucleotides; Accept ‘Polynucleotide’
2. Each nucleotide formed from deoxyribose, a phosphate (group) and an organic/nitrogenous base;
3. Phosphodiester bonds between adjacent nucleotides;
4. A double helix with two polynucleotide chains held together by hydrogen bonds between specific complementary base pairs;
5. Hydrogen bonds between adenine, thymine and cytosine, guanine;

Question 4

Name the types of RNA molecule on the spec.

Answer 4

mRNA - messenger RNA
tRNA - transfer RNA
rRNA - ribosomal RNA

There is also RNAi - RNA interference - a process rather than a molecule, wherein ‘interfering’ RNA molecules bind to mRNA molecules, preventing their translation.

Question 5

Contrast RNA with DNA

Answer 5

RNA: single stranded
DNA: double stranded

RNA: uracil
DNA thymine

RNA: ribose
DNA: deoxyribose

RNA: relatively short polynucleotide chain
DNA: relatively long polynucleotide chain

Question 6

Contrast mRNA with tRNA

Answer 6

tRNA has a specific ‘clover leaf’ shape;
tRNA all the same standard length;
tRNA has an amino acid binding site;
tRNA has one anticodon available (mRNA has many codons);
tRNA has hydrogen bonding between base pairs;

Question 7

Describe the process of semi-conservative replication of DNA.

Answer 7

1. DNA helicase unwinds the DNA double helix
2. DNA helicase breaks hydrogen bonds between complementary bases in the polynucleotide strands
3. Both strands act as templates
4. Free DNA nucleotides attracted to exposed bases on template strands line up in complementary pairs, adenine to thymine and guanine to cytosine
5. DNA polymerase catalyses condensation of phosphodiester bonds between adjacent nucleotides of new strand
6. Each new DNA molecule consists of one old/original/template strand and one new strand

Question 8

Explain how the structure of DNA is related to its functions.

Answer 8

1. Sugar-phosphate (backbone) / double stranded / helix so provides strength / stability / protects bases / protects hydrogen bonds;
2. Long / large molecule so can store lots of information;
3. Helix / coiled so compact / can store in a small amount of space
4. Base sequence allows information to be stored / base sequence codes for amino acids / protein (Accept: base sequence allows transcription)
5. Double stranded so replication can occur semi-conservatively / strands can act as templates / complementary base pairing / A-T and G-C so accurate replication / identical copies can be made;
6. (Weak) hydrogen bonds for replication / unzipping / strand separation / many hydrogen bonds so stable / strong;