1.5 - Nucleic acids

Question 1

(a)

The structure of nucleotides

Answer 1

The general structure of a nucleic acid consists of a pentose sugar a phosphate group and a nitrogenous base

Question 2

(b)

The importance of chemical energy in biological processes

Answer 2

In biological systems it is chemical energy that makes changes because chemical bonds must make or break for reactions to happen.

Question 3

(c)

ATP as an energy carrier

Answer 3

ATP is called the ‘universal energy currency’ as it is used to provide energy for all biochemical reactions in all living organisms.

Question 4

(c)

ATP and its use in the liberation of
energy for cellular activity

Answer 4

ATP releases energy in one hydrolysis reaction controlled by one enzyme. ATP releases energy in small, usable amounts. ATP travels easily to where it may be used for secretion, muscle contraction, nerve transmission or active transport.

Question 5

(d)

The structure of ATP

Answer 5

Adenine
Ribose
Three phosphate groups

Question 6

(e)

DNA bases

Answer 6

Purines- adenine and guanine
Pyrimidines- cytosine and thymine

Question 7

(e)

DNA structure

Answer 7

DNA is made from one strand of nucleotides linked by hydrogen bonds between the bases to another strand that runs antiparallel to the first.

Question 8

(e)

Complementary base pair rule

Answer 8

Adenine and Thymine
Guanine and Cytosine
This complementary base pairing links the two strands and a double helix is formed.

Question 9

(f)

The similarities and differences in the structure of RNA and DNA

Answer 9

RNA
Ribose sugar
Single stranded
A,U,C,G bases
Short polynucleotides

DNA
Deoxyribose sugar
Double stranded
A,T,C,G bases
Long polynucleotides

Question 10

(f)

Three types of RNA found in cells

Answer 10

Messenger RNA (mRNA)
Ribosomal RNA (rRNA)
Transfer RNA (tRNA)

Question 11

(f)

Function of messenger RNA (mRNA)

Answer 11

Made as a complementary copy of the DNA genetic code in the nucleus during transcription. The molecule length is related to the length of the gene transcribed. It attaches to a ribosome in the cytoplasm.

Question 12

(f)

Function of ribosomal RNA (rRNA)

Answer 12

Forms ribosomes.

Question 13

(f)

Function of transfer RNA (tRNA)

Answer 13

carries an amino acid at the 3’ end and an anticodon arm to attach to the mRNA.

Question 14

(g)

Major function of DNA; replication

Answer 14

DNA comprises two complementary strands, the base sequence of one strand determining the base sequence of the other. If two strands of a double helix are seperated, two identical double helices can be formed, as each parent strand acts as a template for the synthesis of a new complementary strand.

Question 15

(g)

Major function of DNA; protein synthesis

Answer 15

The sequence of bases represents the information carried in DNA and determines the sequence of amino acids in proteins.

Question 16

(g)

How DNA mutations can affect protein synthesis

Answer 16

A mutation means the base sequence in DNA is different. This will lead to different MRNA being produced. As a result, the protein produced may have a different structure and lose its function.

Question 17

(h)

Semi-conservative replication of DNA

Answer 17

The replication of DNA to produce two new DNA molecules which both contain one new strand and one old strand from the original DNA molecule.

Question 18

(h)

Role of DNA polymerase in semi-conservative replication

Answer 18

It catalyses the formation of phosphodiester bonds between nucleotides during the synthesis of a new DNA strand.

Question 19

(h)

Role of DNA helicase in semi-conservative replication

Answer 19

It catalyses the unzipping of double-stranded DNA into two single strands, each of which acts as a template.

Question 20

(i)

Genetic code

Answer 20

The genetic code is a linear, universal code for the production of polypeptides.

Question 21

(j)

The triplet code for amino acids

Answer 21

A specific sequence of three nucleotides (codon) on a molecule of DNA or RNA codes for a particular amino acid in protein synthesis.

Question 22

(k)

Exons

Answer 22

Regions of DNA that contain the code for proteins and between the exons are regions of non-coding DNA called introns

Question 23

(l)

Transcription

Answer 23

First stage of protein synthesis and the formation of pre-mRNA in eukaryotes and mRNA in prokaryotes from a section of the template strand of DNA

Question 24

(l)

Transcription process

Answer 24

1. DNA helicase unwinds section of DNA, breaking hydrogen bonds between the DNA strands. Antisense strand acts as a template.
2. RNA polymerase binds to promoter region on a gene
3. Free RNA nucleotides align next to their complementary bases
4. RNA polymerase joins adjacent RNA nucleotides, forming phosphodiester bonds
5. RNA polymerase reaches stop codon and detaches. mRNA complete.

Question 25

# (m) Translation

Answer 25

Second stage of protein synthesis which takes place in the ribosomes. mRNA used as a template for the attachment of tRNA molecules with complementary anticodons. Amino acids carried on adjacent tRNA molecules are joined to form a polypeptide chain

Question 26

# (m) Translation process

Answer 26

1. mRNA attaches to groove between subunits of ribosome 2. Ribosome moves along mRNA until ‘start’ codon reached 3. Amino acid-tRNA complex anticodon attaches to complementary mRNA codon via hydrogen bonding. Another complex binds 4. Peptide bond forms between adjacent amino acids in the complexes 5. Ribosome moves along one codon and release empty tRNA. Process continues to form polypeptide chain until ‘stop’ codon is reached.

Question 27

# (n) The 'one gene - one polypeptide' hypothesis

Answer 27

The theory that each gene encodes a single protein.

Question 28

# (o) The further modification and combination of some polypeptides

Answer 28

After translaton, polypeptides are further modified by adding carbohydrates, lipids or phosphates and different polypeptides may be combined.