Genetics revision notes

Benefits of Asexual Reproduction

Asexual reproduction only requires one parent, which eliminates the need to find a mate. This saves time and energy, allowing organisms to reproduce efficiently even when alone or in isolated populations. The reproductive cycle is often very rapid, enabling a quick increase in population numbers and fast colonisation of new habitats. All offspring are genetically identical to the parent, meaning desirable traits are reliably passed on.

Limitations of Asexual Reproduction

The primary limitation of asexual reproduction is the lack of genetic variation among offspring. Since they are clones, they share the exact same genetic vulnerabilities. If environmental conditions change, or a new disease or predator emerges, an entire asexually reproducing population could be wiped out because no individuals possess advantageous variations to cope with the new challenge. This reduces adaptability and long-term survival prospects in unstable environments.

Genetic Code and Amino Acid Sequence

The order of bases (A, T, C, G) in a DNA sequence determines the order of amino acids in a protein. This information flow goes from DNA to mRNA (transcription) and then to protein (translation). Each set of three bases on mRNA, called a codon, specifies one particular amino acid. The accuracy of this sequence is fundamental for proper protein structure.

Protein Folding and Function

After amino acids are linked into a polypeptide chain, they spontaneously fold into a specific three-dimensional (3D) shape. This folding is dictated by the unique sequence of amino acids and the chemical interactions (e.g., hydrogen bonds, hydrophobic interactions) between them. The specific 3D shape, particularly for enzymes, is essential for its biological function, as it forms the active site for substrate binding.

The Central Dogma

The process of genetic information transfer from DNA to RNA to protein is often referred to as the central dogma of molecular biology. This fundamental concept explains how genetic instructions are converted into functional products within a cell. Any alteration in the DNA base sequence can lead to a change in the amino acid sequence, potentially affecting the final protein's shape and thus its function.

DNA as a Polymer

DNA (Deoxyribonucleic Acid) is a large biological polymer. Polymers are long molecules made up of many repeating smaller units. In the case of DNA, these smaller units are called nucleotides. The polymeric nature allows DNA to store vast amounts of genetic information in a compact form within cells.

Double Helix Structure

DNA consists of two strands that are coiled around each other, forming a shape known as a double helix. This structure can be compared to a twisted ladder. The two strands run in opposite directions and are antiparallel relative to each other.

Complementary Base Pairing

The two DNA strands are linked by specific pairs of chemical bases: adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). These are known as complementary base pairs. These pairings are crucial for DNA replication and protein synthesis due to their specific hydrogen bonding.

Hydrogen Bonds

The complementary base pairs (A-T and G-C) are held together by weak hydrogen bonds. While individual hydrogen bonds are weak, the cumulative effect of many such bonds along the entire DNA molecule provides significant stability to the double helix structure. Their weakness also allows the strands to 'unzip' during replication and transcription.

Nucleotide Composition

Each DNA nucleotide is composed of three parts: a deoxyribose sugar molecule, a phosphate group, and one of four nitrogenous bases (adenine, thymine, guanine, or cytosine). The sugar and phosphate groups form the 'backbone' of the DNA strand, with the bases projecting inwards.

Multiple Alleles in ABO Blood Groups

Blood group inheritance involves multiple alleles, meaning there are more than two possible alleles for a single gene in the population. Specifically, there are three alleles responsible for the ABO blood types: Iᔧ (produces A antigen), Iᔥ (produces B antigen), and i (produces no antigen).

Codominance and Dominance in ABO Blood Groups

The relationship between these alleles demonstrates both codominance and simple dominance. Alleles Iᔧ and Iᔥ are codominant, meaning that if an individual inherits both, both antigens (A and B) are expressed, resulting in blood type AB. Both Iᔧ and Iᔥ are dominant over the i allele, meaning that if i is paired with either Iᔧ or Iᔥ, only the A or B antigen will be expressed, respectively. The i allele is recessive, so individuals with genotype ii have blood type O.

Genotypes and Phenotypes of ABO Blood Groups

The possible genotypes and their corresponding phenotypes are: IᔧIᔧ or Iᔧi result in blood type A. IᔥIᔥ or Iᔥi result in blood type B. IᔧIᔥ results in blood type AB. ii results in blood type O.