Chapter 15, Problem 40A

Summary Introduction

To explain:

The process of occurrence of mutations and the consequences of point mutation and frameshift mutation.

Introduction:

A permanent change in the cell’s DNA is called mutation. This change occurs in the nucleotide sequence in DNA. Mutations can occur in single base pair or a large section of DNA may be deleted. There are many types of mutations; point mutations, insertions and deletions.

Answer to Problem 40A

Mutations are caused by errors in replication, transcription, cell division or by some external agents.

Point mutation- It occurs when one base pair is altered. They are of two types; missense and nonsense. Missense code for wrong amino acid and nonsense change the codon for amino acid to stop codon. Nearly all nonsense mutations lead to proteins that cannot function normally.

Frameshift mutation- Insertion and deletions of a nucleotide causes a shift in frame of the amino acid sequence.

These mutations often lead to many diseases and disorders. Some mutations may be advantageous to an organism as it may help it to survive in harsh conditions better. Mutations provide new variations and these variations are involved in natural selection.

Explanation of Solution

Point mutation- A point mutation is one that involves a chemical change in just one base pair. Substitution is a type of point mutation. This occurs when one base is exchanged for another base. Most substitutions are missense mutations, where the genetic code is altered so that it codes for the wrong amino acid. Other substitutions called nonsense mutations change the codon for an amino acid to a stop codon. These cause translation to stop immediately.

Frameshift mutation- Insertions occur when a nucleotide is added to the DNA sequence. Deletion occurs when a nucleotide is removed from a DNA sequence.These types of mutations change the multiples of three codons from the point of insertion/ deletion. Hence they are called frame-shift mutation because they change the “frame” of the amino acid.

The cumulative effect of mutations in a population might cause a change in allelic frequencies, and thus violate genetic equilibrium. Many mutations are lethal or harmful but some may be good for the organism. This mutation will be selected for and become more common in subsequent generations. In this way, mutations provide a raw material upon which natural selection works.