Unit 1

(a) Mutations are changes in the DNA that can result in no protein or an altered protein being synthesised. 

(b) Single gene mutations involve the alteration of a DNA nucleotide sequence as a result of the substitution, insertion or deletion of nucleotides. 

Substitution mutations can lead to:

• Missense mutations which  result in one amino acid being changed for another. This may result in a non-functional protein or have little effect on the protein. 

• Nonsense mutations which result in a premature stop codon being produced which results in a shorter protein.

• Splice-site mutations which result in some introns being retained and/or some exons not being included in the mature transcript 

Nucleotide insertions or deletions result in frame-shift mutations:

• cause all of the codons and all of the amino acids after the mutation to be changed. 

• This has a major effect on the structure of the protein produced. 

(c) Chromosome structure mutations:

• Duplication is where a section of a chromosome is added from its homologous partner. 

• Deletion is where a section of a chromosome is removed. 

• Inversion is where a section of chromosome is reversed.

• Translocation is where a section of a chromosome is added to a chromosome, not its homologous partner. 

The substantial changes in chromosome mutations often make them lethal. 

Key Area 6: Metabolic Pathways

(a) Metabolic pathways are integrated and controlled pathways of enzyme-catalysed reactions within a cell. Metabolic pathways can have reversible steps, irreversible steps and alternative routes. 

Reactions within metabolic pathways can be anabolic or catabolic. 

• Anabolic reactions build up large molecules from small molecules and require energy. 

• Catabolic reactions break down large molecules into smaller molecules and release energy. 

(b) Metabolic pathways are controlled by the presence or absence of particular enzymes and the regulation of the rate of reaction of key enzymes.

Induced fit occurs when the active site changes shape to better fit the substrate after the substrate binds. 

The substrate molecule(s) have a high affinity for the active site and the subsequent products have a low affinity allowing them to leave the active site. 

When the substrate binds to the active site, this lowers the activation energy (activation energy is the quantity of energy required to start the chemical reaction).

The effects of substrate and product concentration on the direction and rate of enzyme reactions:

• Some metabolic reactions are reversible and the presence of a substrate or the removal of a product will drive a sequence of reactions in a particular direction. 

Control of metabolic pathways through competitive, non-competitive and feedback inhibition of enzymes:

• Competitive inhibitors bind at the active site preventing the substrate from binding. Competitive inhibition can be reversed by increasing substrate concentration. 

• Non-competitive inhibitors bind away from the active site but change the shape of the active site preventing the substrate from binding. Non-competitive inhibition cannot be reversed by increasing substrate concentration.

• Feedback inhibition occurs when the end product in the metabolic pathway reaches a critical concentration. The end-product then inhibits an earlier enzyme, blocking the pathway, and so prevents further synthesis of the end-product. 

(a) Lactate metabolism. 

During vigorous exercise, the muscle cells do not get sufficient oxygen to support the electron transport chain. Under these conditions, pyruvate is converted to lactate. 

This conversion involves the transfer of hydrogen ions from the NADH produced during glycolysis to pyruvate in order to produce lactate. This regenerates the NAD needed to maintain ATP production through glycolysis. 

Lactate accumulates and muscle fatigue occurs. The oxygen debt is repaid when exercise is complete. This allows respiration to provide the energy to convert lactate back to pyruvate and glucose in the liver. 

(b) Types of skeletal muscle fibres.

Most human muscle tissue contains a mixture of both slow- and fast-twitch muscle fibres. Athletes show distinct patterns of muscle fibres that reflect their sporting activities. 

Slow-twitch muscle fibres contract relatively slowly, but can sustain contractions for longer. They are useful for endurance activities such as long-distance running, cycling or cross-country skiing. 

• Slow-twitch muscle fibres rely on aerobic respiration to generate ATP 

• have many mitochondria

• a large blood supply

• a high concentration of the oxygen-storing protein myoglobin. 

• the major storage fuel of slow-twitch muscle fibres is fats. 

Fast-twitch muscle fibres contract relatively quickly, over short periods. They are useful for activities such as sprinting or weightlifting. 

• Fast-twitch muscle fibres can generate ATP through glycolysis only 

• have fewer mitochondria 

• a lower blood supply compared to slow-twitch muscle fibres. 

• The major storage fuel of fast-twitch muscle fibres is glycogen. 

Key Terms to Learn