Principles of Genetic Variation Study Notes

Cells and Chromosomes

Immediately after fertilisation, which occurs when a sperm cell joins an ovum, most organisms consist of a single cell. This single fertilised egg cell contains all the necessary instructions to grow into a multicellular organism, determine how that organism carries out life functions, and even establish susceptibility to certain diseases. The development from a single cell into an average adult human comprising an estimated $50-100$ trillion cells is highly complex.

The Genetic Code and DNA

• Genetic Code: These are the instructions for building and operating an organism.

• DNA: The molecule where the genetic code is found. DNA occurs as chromosomes contained within the nucleus of each cell.

• Nucleus: A central organelle that contains DNA and controls what information leaves the nucleus to enter the cell.

• Chromosomes: These are lengths of DNA and proteins that hold the DNA together.

• Hierarchy of biological organization (Largest to Smallest):

  1. Organism

  2. Cell

  3. Nucleus

  4. Chromosome

  5. DNA

Human Chromosomes

Each species has a unique number of chromosomes. Humans have $46$ chromosomes in most body cells, occurring as $23$ pairs. These are called homologous pairs (from the Greek 'homos' meaning 'the same').

• One chromosome of each pair is inherited from the mother ($23$ chromosomes).

• The other chromosome of the pair is inherited from the father ($23$ chromosomes).

• Homologous chromosomes are identical in size and shape.

Genes and Alleles

Proteins are the 'building blocks' for all organisms and carry out essential biochemical functions. The instructions for making these proteins are carried by the base sequence of DNA.

Definitions

• Gene: A section of DNA that carries the code for making a specific protein. A single gene may be thousands of base pairs long and codes for a specific characteristic or condition (a trait).

• Allele: Alternative versions of genes. Alleles have a slightly different order of bases, meaning they produce different proteins and therefore result in different traits (characteristics).

Relationship in Homologous Pairs

Each pair of homologous chromosomes carries the same genes, but the alleles are not necessarily identical. For example, a homologous pair might both contain the gene for eye colour at the same location, but one might be a 'brown-eyed' version (allele) inherited from the mother and the other a 'blue-eyed' version (allele) from the father.

DNA Replication

To ensure that every new cell has a complete copy of the genetic code, DNA must be copied (replicated) before a cell divides. This process is known as semi-conservative replication because one half of the new DNA molecule is an intact strand from the original parent DNA.

The Process of Replication

1. Unwinding: An enzyme unwinds the parent DNA strand by breaking the hydrogen bonds between bases, exposing the base pairs.

2. Base Pairing: New nucleotides are brought in and bond with the bases on the parent DNA molecule according to the complementary base-pairing rules.

3. Bonding: Enzymes bond the nucleotides together to form the new strand.

4. Result: Two new molecules of DNA are produced, both of which are exact copies of the original. Each winds up to form a helix.

Cell Division: Mitosis and Meiosis

Mitosis

Mitosis is the process of cell division that forms two new daughter cells for the primary purpose of growth and tissue repair.

• Parent and Daughter Cells: It produces two identical daughter cells from a parent cell.

• Chromosome Count: Human body cells and the daughter cells produced by mitosis both contain $46$ chromosomes.

• Occurrence: It occurs in every cell in the body (e.g., skin, growing tissue, repairing tissue) except for gametes.

• Stages:

  • Phase 1: Chromosomes are not visible when the cell is not dividing (protein synthesis is occurring).

  • Phase 2: DNA replicates and condenses into visible chromosomes (two chromatids held by a centromere); the nuclear membrane disappears.

  • Phase 3: Chromosomes line up randomly at the cell equator; a network of fibers forms.

  • Phase 4: Chromatids are separated by the fibers and moved toward opposite poles.

  • Phase 5: Fibers retract, nuclear membranes form, and chromosomes uncoil.

  • Phase 6: Two identical daughter cells are produced.

Meiosis

Meiosis (from Greek 'to make smaller') is a special type of cell division that produces male and female gametes (sperm in males and ova in females).

• Location: Occurs only in the testes (males) and ovaries (females).

• Chromosome Count: It reduces the chromosome number by half. A human body cell has $46$ chromosomes, while a gamete has $23$.

• Outcome: Produces four daughter cells (gametes), each genetically different from the parent cell and from each other.

• Purpose: To ensure that after fertilisation, the resulting zygote has the correct number of chromosomes ($46$).

Variation

Variation refers to the differences among members of the same species (e.g., humans belong to the species Homo sapiens).

Types of Variation

• Continuous Variation: Characteristics that show a complete range of measurements from one extreme to another (e.g., height, weight, beak length, wingspan, growth rate).

  • Large populations usually show a normal distribution, characterized by a bell-shaped graph where extreme characteristics are rare and most individuals are near the average.

• Discontinuous Variation: Characteristics that fall into distinct classes or categories with no in-between (e.g., ear lobes 'fixed' vs. 'free', eye colour, blood type, gender, number of toes, tongue rolling).

Causes of Variation during Meiosis

1. Crossing Over: Homologous chromosomes pair up and exchange sections of DNA (and alleles). This ensures different combinations of alleles in gametes.

2. Independent Assortment: When homologous pairs line up at the cell equator, they do so randomly. It is random which combination of alleles ends up in a particular gamete. In humans, this allows for over $8$ million possible combinations.

Fertilisation

When a sperm ($23$ chromosomes) reaches an ovum ($23$ chromosomes), they combine to form a zygote ($46$ genetically unique chromosomes). This unique arrangement of alleles makes the offspring genetically different from parents and siblings.

Mutation

A mutation is a change in the base sequence (the $A-T-C-G$ sequence) of a section of DNA. It is the ultimate source of variation because it can create entirely new alleles.

Causes and Inheritance

• Mutagens: Agents that increase the rate of mutation, such as radiation (X-rays, radioactive materials), ultraviolet (UV) light, and chemicals (found in cigarettes or fatty foods).

• Somatic (Body) Cell Mutations: Not passed to offspring; other cells usually compensate for the damaged cell.

• Gametic (Sex) Cell Mutations: Occur in sperm or ova and are inherited by offspring, meaning all of the offspring's cells will carry the mutation.

Effects of Mutation

• Silent/Neutral: No observable effect on the organism.

• Harmful: Negatively affects survival. For example, cancer is caused by mutations leading to rapid, abnormal cell division and tumour growth.

• Beneficial: Gives a survival advantage (e.g., poison resistance in mice).

Case Study: Warfarin Resistance in Mice

Warfarin is a chemical used to kill rodents by preventing blood clotting. Some mice developed resistance due to a previously silent mutation in a gene. Resistant mice survived the poison and passed the genes to their offspring. Their quick reproductive time allowed the resistant allele to spread rapidly through the population.

Comparison of Reproduction Methods

Advantages and Disadvantages

• Asexual: Efficient, allows rapid exploitation of favourable conditions (food/space), and produces genetically superior offspring if the parent is well-adapted. However, a lack of variation means the population may go extinct with environmental changes or new diseases.

• Sexual: Produces variation, allowing the species to be more successful over time as some offspring may be better suited for survival. However, it requires more energy and time to find a mate and breed.

The Genetic Code and Proteins

DNA Structure

DNA is a double helix (twisted ladder). It is made of repeating units called nucleotides. A nucleotide comprises:

1. Phosphate

2. Sugar

3. Base

Complementary Base-Pairing Rule

There are four bases: Adenine ($A$), Thymine ($T$), Cytosine ($C$), and Guanine ($G$). The rules are:

• $A$ always bonds with $T$

• $C$ always bonds with $G$

From DNA to Protein

• Triplet: A group of three bases in a gene. Each triplet carries the code for a specific amino acid.

• Peptide Bonds: Amino acids are joined by peptide bonds to form a long chain called a polypeptide chain.

• Protein: A polypeptide chain folded into a specific shape. Proteins form important structures and enzymes responsible for body functions.