Year 13 Biology Notes

Genetics

Genetics studies genes, heredity, and genetic variation.

Gene Concept

  • DNA contains molecular instructions for life.

  • DNA: double-stranded molecule with a ladder-like appearance.

  • Nucleotide components:

    • Nitrogenous base (cytosine, guanine, adenine, thymine/uracil).

    • Pentose sugar (deoxyribose/ribose).

    • Phosphate molecule.

  • Chargaff's Rules:

    • Purine (A, G) = Pyrimidine (T, C).

    • A = T, C = G.

Roles of DNA

  1. Replication:

    • Enzyme Helicase unzips DNA.

    • Enzyme DNA polymerase adds nucleotides to template strand (5’ to 3’ direction).

    • Leading strand synthesized continuously, lagging strand in Okazaki fragments.

    • DNA Ligase joins Okazaki fragments.

    • Semiconservative: new DNA contains one original and one new strand.

    • RNA polymerase proofreads newly synthesized molecule.

  2. Encoding Information:

    • Genes: sequences of A, T, C, and G. Codon (mRNA) specifies an amino acid.

    • Genes determine amino acid sequence in proteins.

    • Transcription: genes transcribed onto mRNA (Uracil replaces Thymine).

    • Translation: RNA strands translated into proteins.

  3. Recombination and Mutation:

    • Genetic recombination: segments of chromosomes swap.

    • Mutations: changes in DNA sequences. Can be beneficial or lethal.

    • Mutagens: physical/chemical agents causing changes (e.g., UV light).

  4. Gene Expression:

    • DNA controls types of proteins a cell makes via interactions with proteins.

Genes

  • Basic unit of heredity coding for specific proteins.

Chromosomes

  • DNA tightly packed, supported by histone proteins.

  • Humans: 46 chromosomes (22 autosome pairs, 1 sex chromosome pair).

  • Females: XX, Males: XY.

  • Karyotype: illustration of chromosomes in decreasing size order.

Cistron

  • DNA length specifying one polypeptide chain.

Gene Action Regulation - Operon Model (Jacob-Monod Model)

  • Operon: operator, promoter gene, and structural genes.

  • Regulator gene codes for repressor protein.

  • Components:

    • Operator: determines structural gene function; blocked by repressor to prevent transcription.

    • Promoter gene: RNA polymerase binding site.

    • Structural gene: codes for specific protein.

    • Regulator gene: codes for repressor protein blocking RNA polymerase.

  • Operons: inducible (start) or repressible (stop).

  • Lac Operon:

    • Lactose present: binds to repressor, allowing transcription.

    • Lactose absent: repressor binds to operator, preventing transcription.

  • Trp Operon:

    • Tryptophan high: repressor blocks RNA polymerase.

    • Tryptophan low/absent: repressor detaches, allowing transcription.

Protein Synthesis

  • RNA: single-stranded nucleic acid; Uracil instead of Thymine.

Roles of RNA

  • mRNA: copies genetic information during transcription.

  • tRNA: transfers amino acids.

  • rRNA: forms ribosomes, facilitating assembly of amino acids.

Transcription

  • DNA unwinds, mRNA is synthesized by RNA polymerase.

Translation

  • mRNA decoded on ribosomes to produce polypeptide sequence.

  • tRNA transports amino acids to ribosome, pairing anticodons with mRNA codons.

  • Peptide bonds form between amino acids.

  • mRNA moves to another ribosome or degrades.

  • Proteins are passed to the Golgi apparatus for modification and packaging.

Genetic code

  • Codon: triplet code. 64 combinations specify 20 amino acids.

  • Degenerate: each amino acid coded by more than one codon.

  • Unambiguous: each codon specifies only one amino acid.

  • Non-overlapping: minimizes mutation effects

  • Universal: same meaning in almost all organisms.

Central Dogma

  • DNA -> mRNA -> Protein

Protein Structure

  • Primary: amino acid sequence.

  • Secondary: alpha-helices and beta-pleated sheets via hydrogen bonds.

  • Tertiary: 3D shape formed by interactions between R groups.

  • Quaternary: multiple polypeptide chains.

Protein Classes

  • Fibrous: elongated shape, structural support. Low water solubility. Keratin, Collagen.

  • Globular: spherical shape, irregular sequences. High water solubility. Insulin, myoglobin.

    Variation

  • Variation is caused by genotypic differences or environmental factors.

continuous variation:

  • gives smooth grading between the two extremes.

discontinuous variation:

  • blood groups (A, B, AB and O).

Genetic (Inherited) variation o Genetic variations are due to the differences in number or structure of chromosomes or by differences in the genes carried by the chromosomes.

Individuals with multiple sets of chromosomes are called polyploid; many common plants have two or more times the normal number of chromosomes and new species may arise by this type of variation.

Environmental variation

  • are affected by factors such as climate, diet, accidents, culture and lifestyle.

MUTATION

  • A mutation is a change in the DNA sequence either due to the mistakes or due to the environmental factors such as UV light, ionizing radiation and chemical mutagens such as tar from cigarette smoke

Effects of mutation

  • A mutation may be neutral and have no effect.

  • A mutation may sometimes be beneficial.

  • Mutations can be harmful and reduce the survival chances of the organisms.

Types of mutations include:

  • Spontaneous

  • Induced

  • Point

  • Block

Examples of point and block mutation

  • Addition (Insertion)

  • Deletion

  • Duplication

  • Inversion

Recombination and Segregation

These are processes that occur during the formation of sex cells where homologous chromosomes in the nucleus forms doublets and mutually recombine.

  • DNA molecule is broken at the same place in both homologous chromosomes.

  • Recombination (crossing Over)

Genetic Linkage and Mapping

  • Genes that are sufficiently close together on a chromosome will tend to “stick together” and the versions of alleles of those genes that are together on a chromosome will be inherited as a pair. This phenomenon is called genetic linkage.

  • The frequency of recombination events between two genes is used to estimate its relative distances on the chromosome.

DISJUCTION

  • Mitosis produces two genetically identical daughter cells where sister chromatids separate and move to opposite poles of a cell that is ‘pulled’ apart by the spindle fibres. This is disjunction.

  • Non-disjunction is the failure of chromosomes to separate during cell division.

POLYPLOIDY

Organisms which have more than 2 whole sets of chromosomes, are termed as polyploidy
TYPES OF INHERITANCE

  • Multiple Alleles

  • Epistasis Gene Interaction

  • Polygenic Traits

Genetic Engineering

  • refers to the direct manipulation of DNA or genome to alter an organism’s characteristics (phenotype) in a particular way.

Recombinant DNA

  • is sometimes referred to as the chimera DNA since it involves combining two or more strands of DNA from two different organisms in order to create a new strand of DNA

Important Tools for Recombinant DNA

  • Restriction endonuclease: These enzymes cut DNA molecules at specific sites in two ways:

  • Exonucleases: is an enzyme that removes nucleotides from the ends of a nucleic acid molecule.

  • DNA ligase: joins two fragments of DNA by synthesizing the phosphodiester bond.

  • DNA polymerase: synthesizes a new complementary DNA strand of an existing DNA or RNA template

  • Vectors: DNA molecule used as a vehicle to artificially carry foreign genetic material into another cell, where it can be replicated or expressed

  • Host organism: Bacteria E. coli is widely used as a host in recombinant DNA technology since cloning and isolation of DNA inserts is much easier to perform

  • Foreign DNA: The desired DNA segment which is to be cloned is called as DNA insert or foreign DNA.

  • Transgenic organism: it carries recombinant DNA. E.g. Bacteria

Basic steps

  1. Selection and isolation of DNA insert.

  2. Selection of suitable cloning vector.

  3. Introduction of DNA cloning vector to form recombinant DNA molecule.

  4. Recombinant DNA molecule is introduced into a suitable host.

  5. Selection of transformed host cells.

  6. Expression and multiplication of DNA insert in the host (Gene Cloning).

Advantages of GMO

  • Insect resistance.

  • Stronger crops.

  • High yield.

Disadvantage of GMO

  • Allergic reactions.

  • Not 100% environmentally friendly.

  • Lower level of biodiversity.

DNA Cloning

  • DNA cloning is the process of making multiple identical copies of a particular piece of DNA

Application of Recombinant DNA Technology

  • Production of transgenic plants and animals.

  • Production of hormones, vaccines and biofuels.

  • Production of antibiotics and commercially important chemicals.

Dangers of Genetic Engineering

  • Spread of new diseases: new dangerous forms of micro-organisms can be developed through recombinant DNA technique either accidentally or deliberately.

BIOTECHNOLOGY

  • is the use of technologies to alter the characteristics of a particular organism.

Benefits of Biotechnology

  • Fuel the world: Biotech uses biological processes such as fermentation etc.

  • Feed the world: Biotech improves crop insect resistance etc.

POPULATION GENETICS

A population is affected to the four main evolutionary forces namely

  1. Natural Selection

  2. Genetic drift

  3. Mutation

  4. Gene flow (migration)

Gene Pool

Gene pool is the sum of total genes of all the individuals in a population.

Allele Frequency

Represents the frequency of a gene (allele) variation in a population.

Genotype frequency

Refers to the total number of a kind of individuals form a population all of which exhibit similar character with respect to the locus under consideration.

THE HARDY WEINBERG PRINCIPLE

Also referred to as the Hardy-Weinberg equilibrium

Assumptions of the Hardy Weinberg Principle

  • Generations must not overlap

  • No natural selection

  • Mutations are negligible

  • no gene flow between populations (migrate in or out of the population)

  • the population is large (no genetic drift)

  • individuals are mating randomly

Factors that affect Genetic Equilibrium

  • Mutation

  • Recombination during sexual reproduction

  • Genetic Drift

  • Gene Migration (gene flow)

NON-DISJUNCTION

  • It is reduction in allele frequencies caused by drastic reduction in population size called population crash e.g. decrease in cheetah population in Africa due to over-hunting.

  • Immigration results in the addition of new alleles into the existing gene pool and changes the allele frequencies. Degree of changes in allele frequencies depends upon the differences between the genotypes of immigrants and native population.

Natural Selection

Types of Natural Selection

  • It favours the average or normal phenotypes, while eliminates the individuals with extreme expressions is Stabilizing Selection.

  • Individuals at one extreme (less adapted) are eliminated while individuals at other extreme (more adapted) are favoured is Directional Selection.

  • It is a type of natural selection which favours extreme phenotype of certain traits over the average is Disruptive Selection

Speciation:

  • refers to the formation of new and distinct species in the course of evolution which involves the splitting of a single evolutionary lineage into two or more genetically independent lineages

  • Allopatric speciation: occurs when population becomes separated by geographical barriers like mountains.

  • Sympatric speciation: It is the formation of two or more species from a single ancestral species occupy the same geographical area.

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