Genetics And Evolution- Molecular Basis Of Inheritance

Genetics and Evolution

Molecular Basis of Inheritance

Introduction to Molecular Basis of Inheritance

Genetic information is stored and transmitted from one generation to the next through DNA.
DNA, or deoxyribonucleic acid, is a macromolecule composed of nucleotides.
These nucleotides consist of a sugar (deoxyribose), a phosphate group, and a nitrogenous base (adenine, thymine, cytosine, or guanine).

DNA Replication

DNA replication is the process by which DNA makes an exact copy of itself.
It occurs during the S phase of the cell cycle.
The double-stranded DNA unwinds and separates, and complementary nucleotides are added to each strand.
DNA replication ensures that each daughter cell receives a complete set of genetic information.

Transcription

Transcription is the process by which DNA is used as a template to produce mRNA.
It occurs in the nucleus of eukaryotic cells.
RNA polymerase binds to the promoter region of a gene and separates the DNA strands.
Complementary RNA nucleotides are added to the growing mRNA strand.

Translation

Translation is the process by which the mRNA sequence is used to synthesize a protein.
It occurs in the ribosomes in the cytoplasm.
Transfer RNA (tRNA) molecules bring amino acids to the ribosome.
The ribosome reads the mRNA codons and matches them with the appropriate tRNA anticodons.

Genetic Code

The genetic code is the set of rules by which the nucleotide sequence of mRNA is translated into the amino acid sequence of a protein.
It is a triplet code, meaning that each three nucleotides (codon) codes for one amino acid.
There are 64 possible codons, including start and stop codons.

Mutations

Mutations are changes in the nucleotide sequence of DNA.
They can occur spontaneously or be induced by mutagens.
Point mutations involve the substitution, insertion, or deletion of a single nucleotide.
Frameshift mutations occur when nucleotides are inserted or deleted, causing a shift in the reading frame.

Gene Expression

Gene expression is the process by which genes are used to make proteins.
It involves transcription of DNA into mRNA and translation of mRNA into protein.
The regulation of gene expression is essential for proper development and functioning of cells.
Gene expression can be regulated at various levels, including transcriptional and post-translational regulation.

Regulation of Gene Expression

Transcription factors bind to specific DNA sequences to activate or repress transcription.
Epigenetic modifications, such as DNA methylation and histone acetylation, can influence gene expression.
Alternative splicing allows different combinations of exons to be included in mRNA, resulting in different protein isoforms.
miRNAs and siRNAs can bind to mRNA and interfere with translation or target mRNA degradation.

Applications of Molecular Biology

Molecular biology has numerous applications in medicine, agriculture, and biotechnology.
PCR (Polymerase Chain Reaction) allows amplification of specific DNA sequences for genetic testing.
Genetic engineering is used to produce genetically modified organisms (GMOs) with desirable traits.
DNA sequencing is essential for understanding the genetic basis of diseases and for studying evolutionary relationships.

Elongation occurs in three phases

Initiation: RNA polymerase binds to the promoter region of the gene.
Elongation: RNA polymerase moves along the DNA template strand, synthesizing mRNA in the 5’ to 3’ direction.
Termination: RNA polymerase reaches a termination sequence, and mRNA is released. Examples:
In prokaryotes, termination is signaled by specific sequences, such as the rho-independent terminator.
In eukaryotes, termination involves the cleavage of the mRNA and polyadenylation.

Post-transcriptional Modifications

In eukaryotes, mRNA undergoes several modifications before it is ready to be translated.
Addition of a 5’ cap: A modified guanine nucleotide is added to the 5’ end of the mRNA, providing stability and facilitating translation.
Addition of a poly-A tail: A string of adenine nucleotides is added to the 3’ end of the mRNA, protecting it from degradation.
RNA splicing: Introns (non-coding regions) are removed, and exons (coding regions) are joined together.

The Genetic Code

The genetic code is a set of rules that determines the correspondence between each mRNA codon and the corresponding amino acid.
There are 20 amino acids used to build proteins, and the genetic code is degenerate, meaning that multiple codons can code for the same amino acid.
Example: The codon AUG codes for the amino acid methionine and also serves as the start codon. Equation: mRNA codon → Amino acid

Examples of Genetic Disorders

Cystic fibrosis: Caused by a mutation in the CFTR gene, resulting in a defective chloride ion channel.
Huntington’s disease: Caused by an expansion of a trinucleotide repeat in the huntingtin gene, leading to the formation of abnormal protein aggregates.
Sickle cell anemia: Caused by a single nucleotide substitution in the hemoglobin gene, resulting in abnormal hemoglobin structure.

Regulation of Gene Expression

Transcriptional regulation: Transcription factors bind to DNA and can activate or repress gene expression.
Post-transcriptional regulation: miRNAs and siRNAs can bind to mRNA and inhibit translation or cause mRNA degradation.
Translational regulation: Regulatory proteins can bind to the mRNA and inhibit translation initiation.
Post-translational regulation: Protein modifications, such as phosphorylation, can alter protein activity and function.

Epigenetics

Epigenetics refers to changes in gene expression that do not involve alterations in the DNA sequence.
DNA methylation: Addition of a methyl group to cytosine bases can silence gene expression.
Histone modifications: Acetylation and methylation of histone proteins can influence gene expression.
Epigenetic changes can be heritable and play a role in development, aging, and disease.

Types of Mutations

Silent mutations: A change in the DNA sequence that does not result in a change in the corresponding amino acid.
Missense mutations: A change in the DNA sequence that results in a different amino acid being incorporated into the protein.
Nonsense mutations: A change in the DNA sequence that leads to the premature termination of protein synthesis.
Frameshift mutations: Insertion or deletion of nucleotides causes a shift in the reading frame, altering the entire amino acid sequence.

Evolutionary Significance of Mutations

Mutations are the ultimate source of genetic variation, which is essential for evolution to occur.
Beneficial mutations: Mutations that confer a selective advantage to an organism, enabling it to better survive and reproduce.
Neutral mutations: Mutations that have no effect on an organism’s fitness.
Harmful mutations: Mutations that decrease an organism’s fitness and may lead to disease or other negative effects.

Hardy-Weinberg Principle

The Hardy-Weinberg principle describes the relationship between allele frequencies in a population and the genotype frequencies.
In a large population with random mating and no natural selection, the allele frequencies remain constant across generations.
Equations:
- p + q = 1 (for a two-allele system)
- p^2 + 2pq + q^2 = 1 (for genotype frequencies)

Examples of Evolutionary Mechanisms

Natural selection: Differential survival and reproduction of individuals based on their inherited traits.
Genetic drift: Random changes in allele frequencies due to chance events, more pronounced in small populations.
Gene flow: The movement of alleles between populations through migration or transfer of gametes.
Mutation: The ultimate source of genetic variation in a population.

Genetic Engineering

Genetic engineering is the manipulation of an organism’s genetic material to modify its characteristics.
Recombinant DNA technology allows the transfer of genes between different organisms.
Techniques such as gene cloning, PCR, and DNA sequencing are used in genetic engineering.
Applications of genetic engineering include the production of medicines, improvement of crop plants, and genetic modification of bacteria for industrial purposes.

Polymerase Chain Reaction (PCR)

PCR is a technique used to amplify a specific DNA sequence.
It involves cycles of denaturation, annealing, and extension of DNA using a heat-stable DNA polymerase.
PCR can be used in various applications such as DNA fingerprinting, amplification of DNA for sequencing, and detection of infectious agents.
The equation for PCR is: DNA template + Primers + Nucleotides + DNA polymerase → Amplified DNA

DNA Sequencing

DNA sequencing is the process of determining the precise order of nucleotides in a DNA molecule.
It has revolutionized many fields of biology, including genomics and molecular evolution.
Sanger sequencing, pyrosequencing, and next-generation sequencing (NGS) are common methods used for DNA sequencing.
Applications of DNA sequencing include studying genetic diseases, identifying mutations, and understanding evolutionary relationships.

Genomics

Genomics is the study of an organism’s complete set of genes and their functions.
It involves sequencing, mapping, and analyzing the genomes of organisms.
Comparative genomics compares the genomes of different organisms to understand evolutionary relationships and identify genes associated with diseases.
Functional genomics aims to understand the functions of genes and how they interact with each other.

Evolutionary Biology

Evolutionary biology is the study of how species evolve and change over time.
It involves understanding the mechanisms of evolution, such as natural selection, genetic drift, and gene flow.
Evolutionary biologists study fossils, comparative anatomy, molecular genetics, and other evidence to reconstruct the evolutionary history of organisms.
Evolutionary biology has applications in fields such as medicine, conservation biology, and agriculture.

Evidence for Evolution

Fossils: Fossil records show the existence of organisms that lived in the past and provide evidence of evolutionary changes over time.
Comparative anatomy: Similarities in anatomical structures among different species indicate common ancestry.
Embryology: The study of embryonic development reveals similarities in the early stages of different organisms, suggesting a shared evolutionary history.
Molecular biology: DNA and protein sequence comparisons provide evidence of evolutionary relationships.

Modes of Natural Selection

Directional selection: Favors individuals that have an extreme phenotype and shifts the population’s characteristics in one direction.
Stabilizing selection: Favors individuals with intermediate phenotypes and reduces variation in a population.
Disruptive selection: Favors individuals with both extreme phenotypes and can lead to the formation of two distinct populations.
Sexual selection: Selection based on traits that enhance an individual’s chances of mating and reproducing.

Adaptive Radiation

Adaptive radiation is the diversification of a common ancestor into a variety of different species, each adapted to occupy a specific niche.
It often occurs when species colonize new habitats or when environmental conditions change.
Examples of adaptive radiation include Darwin’s finches in the Galapagos Islands and the radiation of mammals after the extinction of dinosaurs.

Speciation

Speciation is the formation of new species from an existing population.
It occurs when gene flow is interrupted, leading to the accumulation of genetic differences between populations.
Allopatric speciation occurs when populations are geographically isolated, while sympatric speciation occurs within the same geographic area.
Reproductive isolation, such as mating preferences or changes in chromosome number, plays a critical role in speciation.

Evolutionary Applications

Conservation biology: Understanding evolutionary relationships helps in conservation efforts and the preservation of endangered species.
Agricultural genetics: Genetic techniques are used to develop crop plants with desirable traits, such as disease resistance and increased yields.
Medicine: Knowledge of evolutionary biology helps in understanding the evolution of pathogens, drug resistance, and the development of vaccines.
Forensic science: DNA analysis and evolutionary principles are used to solve criminal cases and identify human remains.