Genetics and Molecular Biology: Mitosis, Meiosis, and Chromosome Structure

Genetics and Molecular Biology Unit

Introduction to Genetics and Molecular Biology

Genetics and molecular biology are foundational disciplines in biology, focusing on the inheritance and expression of traits. Genetics studies how observable traits are passed from one generation to the next and their effects on populations and species. Molecular biology examines the molecular processes that transfer genetic information from genotype to phenotype.

• Genetics: The study of inheritance of observable traits (e.g., eye color, genetic diseases).

• Molecular Biology: The study of molecular processes involved in the transfer of genetic information from genotype to phenotype.

• Nature vs Nurture: Some traits are determined solely by genetics (e.g., ABO blood groups), while others are influenced by environmental factors (e.g., hydrangea flower color).

Genotype and Phenotype

The genotype is the genetic makeup of an organism, while the phenotype is the observable physical and biochemical traits.

• Genotype: The genetic information contained in genes.

• Phenotype: The physical and biochemical traits expressed by the genotype.

• Example: Flower color, ear shape, and genetic diseases are phenotypes determined by specific genotypes.

DNA, Chromosomes, and Genes

Structure of DNA

DNA is the hereditary material in all living organisms. Its structure is a double helix composed of a sugar-phosphate backbone and nitrogenous bases.

• Sugar-phosphate backbone: Forms the 'ribbons' of the DNA molecule.

• Nitrogenous bases: Form the 'rungs of the ladder' and pair via hydrogen bonds (A-T, G-C).

• Double helix: The overall three-dimensional structure of DNA.

DNA Sequences and Bioinformatics

DNA sequences are represented as strings of nucleotide letters (A, T, G, C) in databases and bioinformatics tools, not as graphical models.

• Example: Homo sapiens myoglobin gene sequence: 'gtact catga...'

Chromosome Structure

Each eukaryotic chromosome is a long DNA molecule with distinct regions:

• Short arm (p arm): The shorter section of the chromosome.

• Long arm (q arm): The longer section of the chromosome.

• Centromere: The constricted region where sister chromatids are joined.

• Telomere: The end region of the chromosome, protecting it from degradation.

Homologous Chromosomes and Alleles

Homologous chromosomes are pairs with the same size, shape, and genes, but may have different alleles (versions of a gene).

• Locus: A specific location on a chromosome where a gene is found.

• Allele: Alternative versions of a gene (e.g., red eye allele vs. white eye allele in fruit flies).

• Homologous chromosomes: Not identical; each may carry different alleles at the same locus.

Sister Chromatids vs. Homologous Chromosomes

Sister chromatids are identical copies of a chromosome, joined at the centromere, formed during DNA replication. Homologous chromosomes are similar but not identical.

• Sister chromatids: Identical nucleotide sequences, present only after DNA replication (S phase).

• Homologous chromosomes: Same genes, possibly different alleles, present in G1 phase before replication.

Cell Cycle and Chromosome Dynamics

Phases of the Cell Cycle

The cell cycle consists of several phases: G1, S, G2, and M (mitosis). Chromosomes undergo replication and segregation during these phases.

• G1 phase: Cell growth, homologous chromosomes present.

• S phase: DNA replication, formation of sister chromatids.

• G2 phase: Preparation for mitosis, chromosomes consist of sister chromatids.

• M phase (Mitosis): Division of the nucleus and segregation of chromosomes.

Mitosis: Stages and Purpose

Mitosis is the process by which a cell divides to produce two genetically identical daughter cells. It ensures the faithful transmission of genetic material.

• Prophase: Chromosomes condense, nuclear envelope breaks down.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Nuclear envelope reforms, chromosomes decondense.

• Purpose: To ensure each daughter cell receives an exact copy of the parent cell's DNA.

Chromosome Number and Karyotyping

Diploid and Haploid Cells

Somatic cells are diploid (2n), containing two sets of homologous chromosomes. Sex cells (gametes) are haploid (n), containing one set.

• Diploid (2n): Two sets of chromosomes (e.g., human somatic cells).

• Haploid (n): One set of chromosomes (e.g., sperm and egg cells).

Human Karyotype

A karyotype is a display of condensed chromosomes arranged in pairs, used to study chromosome number and structure.

• Application: Used in genetics to identify chromosomal abnormalities.

• Technique: Chromosomes are stained (e.g., Giemsa stain) and visualized under a microscope.

Cell Type	Chromosome Number	Example
Somatic Cell	Diploid (2n)	Human body cell
Gamete	Haploid (n)	Sperm or egg

Organization of DNA

Chromatin Structure

DNA is packaged with proteins called histones to form chromatin, which exists in several conformations in the nucleus.

• Histones: Proteins that help package DNA into nucleosomes.

• Chromatin: DNA-protein complex; can be euchromatin (active) or heterochromatin (inactive).

• Nucleosome: Fundamental unit of chromatin, consisting of DNA wrapped around histone proteins.

Chromatin Type	Activity	Description
Euchromatin	Transcriptionally active	Less condensed, accessible to transcription machinery
Heterochromatin	Transcriptionally inactive	Highly condensed, not accessible

Levels of DNA Organization

• Double helix: 2 nm diameter

• Nucleosome: 10 nm diameter

• Chromatin fiber: 30 nm diameter

• Loops and scaffolds: 300 nm fiber

• Replicated chromosome: 1400 nm

Additional info: Chromatin structure regulates gene expression by controlling access to DNA for transcription.