The Genetic Code

In this guide, we will explore the concept of the genetic code, including DNA structure and base sequences, gene switching, mutations, and monohybrid crosses. These concepts are fundamental to understanding how genetic information is inherited and how it affects organisms.

DNA Structure and Base Sequences

DNA (Deoxyribonucleic Acid) is the molecule that contains the genetic instructions for the development, functioning, and reproduction of living organisms. The structure of DNA is crucial for its function, and understanding how the base sequences work is essential to grasping how genetic information is encoded.

Structure of DNA

DNA is made up of two long chains (or strands) coiled into a double helix. Each strand is made up of smaller units called nucleotides. A nucleotide consists of:

A phosphate group.

A deoxyribose sugar.

A nitrogenous base.

There are four types of nitrogenous bases in DNA:

• Adenine (A).
• Thymine (T).
• Cytosine (C).
• Guanine (G).

The bases pair in a specific way:

• Adenine pairs with Thymine (A-T).
• Cytosine pairs with Guanine (C-G).

These base pairs are held together by hydrogen bonds, and the order of these bases along the DNA strand forms the genetic code. The sequence of bases along a DNA strand determines the sequence of amino acids in a protein, which in turn determines the protein’s function.

Gene Switching

Gene switching, or gene regulation, refers to the process by which certain genes are turned on or off, depending on the needs of the cell or organism. This process is essential for controlling the production of proteins in different tissues and at different times during development.

How Gene Switching Works

• Regulatory Genes: Some genes produce proteins that regulate other genes. These regulatory proteins can bind to specific sequences of DNA near the gene they control, either activating or repressing the gene's expression.
• Promoters and Enhancers: In DNA, regions called promoters help initiate the process of gene expression, while enhancers can increase the activity of a gene. Both are crucial for the precise control of gene switching.
• Environmental Factors: Gene expression can also be influenced by environmental factors such as temperature, diet, or exposure to chemicals. For example, the presence of certain hormones can turn genes on or off, influencing growth, development, and metabolism.

Gene switching is essential for development, as it allows cells to specialise and function differently in various parts of the body, despite having the same DNA. For example, a skin cell and a muscle cell have the same DNA, but different genes are expressed in each type of cell, giving them their distinct functions.

Mutations

A mutation is a change in the sequence of bases in an organism's DNA. Mutations can occur naturally, but they can also be caused by environmental factors such as radiation or chemicals. Mutations are a source of genetic variation, and while most mutations have little effect, some can lead to significant changes in an organism’s characteristics.

Types of Mutations

Point Mutation: A single base in the DNA sequence is replaced by another base. This can lead to a change in one amino acid in a protein, potentially affecting its function.

• Example: Sickle cell anaemia is caused by a point mutation where adenine is replaced by thymine in the gene for haemoglobin.

Insertion or Deletion: One or more bases are added or removed from the DNA sequence. This can result in a frameshift mutation, where the reading frame of the genetic code is altered, potentially changing the entire protein.

Duplication: A section of DNA is duplicated, which can lead to multiple copies of a particular gene.

Silent Mutation: Some mutations do not change the amino acid sequence of the protein, due to the redundancy of the genetic code (where different codons can code for the same amino acid).

Effects of Mutations

Mutations can have various effects on an organism:

• Beneficial Mutations: In some cases, mutations can be beneficial and provide an advantage, such as antibiotic resistance in bacteria or adaptations to environmental changes.
• Harmful Mutations: Other mutations can cause diseases or disorders, such as cystic fibrosis, which is caused by a mutation in the gene that codes for a protein involved in the movement of salt across cell membranes.
• Neutral Mutations: Many mutations have no significant effect, particularly if they occur in non-coding regions of DNA.

Monohybrid Crosses

A monohybrid cross is a type of genetic cross that examines the inheritance of a single trait. It involves two parents that differ in a single characteristic, and is often used to understand how specific alleles are passed from one generation to the next.

Alleles and Dominance

• Alleles: Different forms of a gene are called alleles. An individual has two alleles for each gene, one inherited from each parent. For example, the gene for flower colour in pea plants can have a purple (P) allele or a white (p) allele.
• Dominant and Recessive Alleles: In many cases, one allele can be dominant over the other. A dominant allele (e.g., P for purple flowers) will express its trait even if only one copy is present, whereas a recessive allele (e.g., p for white flowers) will only express its trait if both alleles are recessive (pp).

Punnett Square

A Punnett square is a tool used to predict the possible genetic outcomes of a monohybrid cross. It shows how alleles from each parent combine to produce offspring with different genotypes (the genetic makeup).

Example: Cross between two heterozygous purple-flowered pea plants (Pp × Pp)

In this case:

• PP and Pp offspring will have purple flowers, because the purple allele (P) is dominant.
• pp offspring will have white flowers, as both alleles must be recessive for this trait to be expressed.

Genotypic and Phenotypic Ratios

• Genotypic ratio: The ratio of different genotypes (e.g., 1 PP : 2 Pp : 1 pp).
• Phenotypic ratio: The ratio of different physical traits (e.g., 3 purple : 1 white).

Summary

The genetic code is the set of instructions encoded in the DNA that guides the development and function of an organism. DNA's structure and base sequences form the foundation of this code, while gene switching ensures the right genes are expressed at the right times. Mutations provide genetic diversity, which can lead to beneficial or harmful effects. Monohybrid crosses are a simple way to predict how traits are inherited, using dominant and recessive alleles. Understanding these concepts is key to unlocking how genetic information shapes all living organisms.