1.5 - Nucleic acids 1.6 - ATP

Draw the structure of a nucleotide.

Name the pentose sugars in DNA &

RNA.

DNA: deoxyribose

RNA: ribose

State the role of DNA in living cells.

Base sequence of genes codes for functional

RNA & amino acid sequence of polypeptides.

Genetic information determines inherited

characteristics = influences structure &

function of organisms.

State the role of RNA in living cells.

mRNA: Complementary sequence to 1 gene from DNA

with introns (non-coding regions) spliced out. Codons

can be translated into a polypeptide by ribosomes.

rRNA: component of ribosomes (along with proteins)

tRNA: supplies complementary amino acid to mRNA

codons during translation

How do polynucleotides form?

Condensation reactions between

nucleotides form strong phosphodiester

bonds (sugar-phosphate backbone).

Describe the structure of DNA.

double helix of 2 polynucleotide strands

(deoxyribose)

H-bonds between complementary purine &

pyrimidine base pairs on opposite strands:

adenine (A) + thymine (T)

guanine (G) + cytosine (C)

Which bases are purine and which are

pyrimidine?

A & G = 2-ring purine bases

T & C & U = 1-ring pyrimidine bases

Name the complementary base pairs in

DNA

2 H-bonds between

adenine (A) + thymine (T)

3 H-bonds between

guanine (G) + cytosine (C)

Name the complementary base pairs in

RNA.

2 H-bonds between

adenine (A) + uracil (U)

3 H-bonds between

guanine (G) + cytosine (C)

Relate the structure of DNA to its

functions.

● sugar-phosphate backbone & many H-bonds provide stability

● long molecule stores lots of information

● helix is compact for storage in nucleus

● base sequence of triplets codes for amino acids

● double-stranded for semi-conservative replication

● complementary base pairing for accurate replication

● weak H-bonds break so strands separate for replication

Describe the structure of messenger

RNA (mRNA).

● Long ribose polynucleotide (but shorter than

DNA).

● Contains uracil instead of thymine.

● Single-stranded & linear (no complementary

base pairing).

● Codon sequence is complementary to exons of 1

gene from 1 DNA strand.

Relate the structure of messenger RNA

(mRNA) to its functions.

NB: functions given in same order as related structural feature on

previous slide

● Breaks down quickly so no excess polypeptide forms.

● Ribosome can move along strand & tRNA can bind to

exposed bases.

● Can be translated into a specific polypeptide by

ribosomes.

Describe the structure of transfer RNA

(tRNA).

● Single strand of about 80 nucleotides.

● Folded into clover shape (some paired bases).

● Anticodon on one end, amino acid binding site

on the other:

a) anticodon binds to complementary mRNA codon

b) amino acid corresponds to anticodon

Order DNA, mRNA and tRNA according

to increasing length.

tRNA

mRNA

DNA

Why did scientists initially doubt that

DNA carried the genetic code?

Chemically simple molecule with few

components.

Why is DNA replication described as

‘semiconservative’?

● Strands from original DNA molecule

act as a template.

● New DNA molecule contains 1 old

strand & 1 new strand.

Outline the process of semiconservative

DNA replication.

1. DNA helicase breaks H-bonds between base pairs.

2. Each strand acts as a template.

3. Free nucleotides from nuclear sap attach to exposed

bases by complementary base pairing.

4. DNA polymerase catalyses condensation reactions that

join adjacent nucleotides on new strand.

5. H-bonds reform.

Describe the Meselson-Stahl

experiment.

1. Bacteria were grown in a medium containing heavy

isotope 15

N for many generations.

2. Some bacteria were moved to a medium containing

light isotope 14

N. Samples were extracted after 1 & 2

cycles of DNA replication.

3. Centrifugation formed a pellet. Heavier DNA (bases

made from 15N) settled closer to bottom of tube.

Explain how the Meselson-Stahl

experiment validated semiconservative

replication.

Describe the structure of adenosine

triphosphate (ATP).

nucleotide

derivative of

adenine with 3

phosphate groups

Explain the role of ATP in cells.

Energy released is coupled to metabolic

reactions.

● Phosphate group phosphorylates

compounds to make them more reactive.

ATP hydrolase catalyses ATP → ADP + Pi

How is ATP resynthesised in cells?

● ATP synthase catalyses condensation

reaction between ADP & Pi

● during photosynthesis & respiration

Explain why ATP is suitable as the

‘energy currency’ of cells.

● High energy bonds between phosphate groups.

● Small amounts of energy released at a time =

less energy wasted as heat.

● Single-step hydrolysis = energy available

quickly.

● Readily resynthesised.