Genetic Variation, DNA Structure, and the Molecular Basis of Inheritance

Genetic Variation in Individuals

Learning Objectives

• Understand the relationships among locus, gene, allele, mutation, protein, haploid, diploid, chromatin, chromosome, base sequence, gene expression, transcription, and translation.

• Explain how genotype is linked to phenotype.

• Describe how alternative splicing increases protein diversity.

• Identify types of point mutations and chromosomal rearrangements and their effects on phenotype.

• Distinguish between mitosis and meiosis and their biological significance.

• Explain how chromosomal segregation during meiosis leads to genetic recombination and altered ploidy.

History and Biological Role of DNA

Chemical Evolution of Life

The origin of life is hypothesized to have begun with the formation of complex organic molecules from simple, non-living elements. Early 20th-century scientists, such as A.I. Oparin and J.B.S. Haldane, proposed that life originated from chemical interactions in Earth's early environment, leading to the formation of molecules like nucleic acids, sugars, and lipids.

• Stanley Miller's Experiment (1953): Simulated early Earth conditions by mixing methane (CH4), ammonia (NH3), hydrogen (H2), and water vapor, then applying electrical sparks to mimic lightning. This resulted in the formation of amino acids and other organic molecules.

• Key Models for the Origin of Life:

  • Prebiotic Soup Model: Organic molecules accumulated in the early oceans.

  • Surface Metabolism Model: Chemical reactions occurred on mineral surfaces, such as those found at deep-sea hydrothermal vents.

  • RNA World Hypothesis: Early life forms may have used RNA for information storage and catalysis before the evolution of DNA and proteins.

Essential Roles of DNA

Functions of DNA in Cells

DNA is the hereditary material in all known living organisms and some viruses. It fulfills several essential roles:

• Information Storage: DNA stores genetic information in the form of genes and alleles.

• Structure and Function: DNA encodes instructions for protein synthesis, determining organismal structure and function.

• Replication: DNA can replicate accurately, enabling growth and development via mitosis.

• Inheritance: DNA is responsible for the transfer of traits from parents to offspring through meiosis and fertilization.

• Evolution: DNA is capable of change through mutations and recombination, providing the raw material for evolution.

Structure of DNA

Primary Structure

• DNA is a polymer made of nucleotides, each consisting of:

  • A phosphate group

  • A five-carbon sugar (deoxyribose in DNA, ribose in RNA)

  • A nitrogenous base (adenine [A], guanine [G], cytosine [C], thymine [T] in DNA; uracil [U] replaces thymine in RNA)

• Nucleotides are linked by phosphodiester bonds, forming a sugar-phosphate backbone with directionality (5' to 3').

Secondary Structure

• DNA exists as a double helix with two antiparallel strands (5' to 3' and 3' to 5').

• Strands are held together by complementary base pairing: A pairs with T, and C pairs with G.

• The double helix is stabilized by hydrogen bonds and base stacking interactions.

Tertiary Structure

• In eukaryotic cells, DNA wraps around histone proteins to form chromatin, which further condenses into chromosomes during cell division.

• Most of the time, DNA is in a less condensed form to allow access for transcription.

Genes, Alleles, and Chromosomes

Genetic Information Organization

• A gene is a segment of DNA at a specific locus on a chromosome that encodes an RNA or protein product.

• Alleles are different versions of a gene found at the same locus.

• Haploid cells (n) have one set of chromosomes (e.g., gametes), while diploid cells (2n) have two sets (e.g., somatic cells).

• Humans have 46 chromosomes (23 pairs), including autosomes and sex chromosomes (XX or XY).

• Cells can be homozygous (same alleles at a locus) or heterozygous (different alleles at a locus).

From Genotype to Phenotype: The Central Dogma

Gene Expression Overview

The Central Dogma of molecular biology describes the flow of genetic information:

• Transcription: DNA is used as a template to synthesize messenger RNA (mRNA).

• Translation: mRNA is used as a template to assemble amino acids into proteins.

$\text{DNA} \xrightarrow{\text{transcription}} \text{mRNA} \xrightarrow{\text{translation}} \text{Protein}$

Transcription

• Occurs in the nucleus (eukaryotes) or cytoplasm (prokaryotes).

• RNA polymerase synthesizes mRNA using the DNA template strand.

• Regulated by transcription factors, activators, and repressors.

• Genes contain exons (coding regions) and introns (non-coding regions); introns are removed by splicing before mRNA exits the nucleus.

• Alternative splicing allows a single gene to produce multiple protein products by varying the combination of exons included in the final mRNA.

Translation

• Occurs in the cytoplasm at ribosomes.

• mRNA codons (three-base sequences) specify amino acids, which are brought by transfer RNAs (tRNAs).

• The genetic code is redundant (multiple codons for one amino acid), unambiguous (each codon codes for only one amino acid), non-overlapping, and nearly universal across organisms.

• Translation proceeds through initiation (start codon recognition), elongation (amino acid addition), and termination (stop codon recognition).

Protein Folding and Function

• After translation, polypeptide chains fold into specific three-dimensional structures, sometimes combining with other polypeptides to form functional proteins (e.g., hemoglobin).

• Protein structure determines function; misfolding can lead to loss of function or disease.

Mutations and Genetic Variation

Types and Consequences of Mutations

• Mutation: A permanent change in the DNA sequence. Can occur spontaneously or be induced by environmental factors (e.g., UV, chemicals).

• Point mutations: Change a single base pair. Types include:

  • Silent mutation: No change in amino acid (due to redundancy in the genetic code).

  • Missense mutation: Changes one amino acid, potentially altering protein function.

  • Nonsense mutation: Introduces a premature stop codon, truncating the protein.

  • Frameshift mutation: Insertion or deletion of bases shifts the reading frame, often resulting in nonfunctional proteins.

• Chromosomal rearrangements: Large-scale changes such as deletions, duplications, inversions, or translocations can disrupt gene function and lead to phenotypic changes.

• Mutations are a source of genetic variation, which is essential for evolution. Harmful mutations may be eliminated by natural selection, while beneficial mutations can spread in populations.

Cell Cycle and Cell Division

Overview of the Cell Cycle

• The cell cycle consists of interphase (G1, S, G2 phases) and the mitotic (M) phase.

• G1 phase: Cell grows and performs normal functions.

• S phase: DNA is replicated.

• G2 phase: Cell prepares for division.

• M phase: Mitosis (nuclear division) and cytokinesis (cytoplasmic division) occur.

• Checkpoints ensure proper progression and DNA integrity.

Mitosis vs. Meiosis

Feature	Mitosis	Meiosis
Cell type	Somatic cells	Germ cells
Chromosome alignment	Single file	Homologous pairs (double file)
Genetic outcome	Genetically identical cells	Genetically diverse cells
Ploidy of daughter cells	Diploid (2n)	Haploid (n)
Number of daughter cells	2	4

Genetic Variation from Meiosis

• Independent assortment: Random distribution of maternal and paternal chromosomes during meiosis I increases genetic diversity.

• Crossing over: Exchange of genetic material between homologous chromosomes during prophase I creates new allele combinations.

• Random fertilization: Further increases genetic variation in sexually reproducing populations.

Significance of Sexual Reproduction

• Although asexual reproduction can produce more offspring quickly, sexual reproduction generates greater genetic diversity, which is advantageous in changing environments.

• Purifying selection hypothesis: Sexual reproduction helps eliminate deleterious mutations through recombination.

• Changing environment hypothesis: Genetic diversity from sexual reproduction increases the likelihood of survival under new environmental pressures.

Errors in Meiosis

• Improper chromosomal segregation can lead to nondisjunction, resulting in gametes with abnormal chromosome numbers (aneuploidy), which can cause genetic disorders.