Genetics Revision Notes

DNA as a Polymer

DNA is a polymer, meaning it is a large molecule made up of repeated smaller units called monomers. In the case of DNA, these monomers are nucleotides. This polymeric structure is essential for storing vast amounts of genetic information.

Double Helix Structure

The DNA molecule consists of two long strands coiled around each other to form a double helix. This iconic structure was described by Watson and Crick and is fundamental to how DNA functions in carrying and transmitting genetic information.

Nucleotide Composition

Each nucleotide, the basic building block of DNA, is comprised of three parts: a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases (adenine, thymine, guanine, or cytosine). The sugar and phosphate form the backbone of each strand.

Complementary Base Pairing

The two strands of DNA are linked by a series of complementary base pairs. Adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). These pairs are held together by weak hydrogen bonds, which allow for easy separation during DNA replication and transcription.

Purpose of Meiosis

Meiosis is a type of cell division that produces gametes (sex cells), like sperm and egg cells, necessary for sexual reproduction. It ensures that offspring inherit genetic material from both parents, maintaining the species' chromosome number, and introduces genetic diversity.

Outcome of Meiosis

A single parent cell undergoes two divisions to produce four daughter cells. These daughter cells are haploid, meaning they contain half the number of chromosomes as the original parent cell. Each of these four daughter cells is genetically distinct due to genetic recombination during meiosis.

Genetic Diversity

One of the key outcomes of meiosis is the creation of genetically distinct gametes. This genetic diversity is vital for the survival and adaptation of a species, as it provides a wider range of traits for natural selection to act upon, contributing to the evolution of populations.

The Purpose of Each Step in DNA Extraction

Mashing/blending physically breaks cell walls. Salt (sodium ions) helps neutralise DNA's negative charge, aiding precipitation. Detergent disrupts lipid membranes, releasing DNA. Filtering removes cellular debris. Cold ethanol/propan-2-2ol precipitates DNA, making it visible.

Chemicals Used and Their Functions

Salt solution: provides positive ions (e.g., sodium) to neutralise the negative charge of DNA, allowing strands to clump. Detergent: emulsifies lipids in cell and nuclear membranes, breaking them down to release DNA. Ethanol/propan-2-ol (cold): causes DNA to become insoluble and precipitate out of the aqueous solution, allowing for its isolation.

Human Sex Chromosomes

Humans typically have 23 pairs of chromosomes, with one pair being the sex chromosomes. Females possess two X chromosomes (XX), while males have one X and one Y chromosome (XY). The remaining 22 pairs are known as autosomes.

Gamete Formation and Fertilisation

During gamete formation (meiosis), females produce eggs that all contain a single X chromosome. Males produce sperm, with about half carrying an X chromosome and the other half carrying a Y chromosome. The sex of the offspring is determined by which type of sperm fertilises the egg. An X-carrying sperm results in a female (XX), and a Y-carrying sperm results in a male (XY).

Genetic Diagram for Sex Determination

A Punnett square or genetic cross diagram illustrates these probabilities. Parental genotypes: Female (XX) and Male (XY). Gametes from female: X, X. Gametes from male: X, Y. Possible offspring genotypes: XX, XY. The phenotypic ratio is approximately 1 female: 1 male, representing a 50% chance for each sex.