Section 2 – Cell Biology & Genetics

Introduction

The cell is the basic structural, functional and organizational unit of living organisms. Every living organism is composed of one or more cells. Cells carry out essential life processes such as metabolism, growth, response to stimuli, reproduction and maintenance of internal organization.

On the basis of organization, cells are broadly divided into prokaryotic cells and eukaryotic cells.

Prokaryotic Cell

Prokaryotic cells are structurally simpler cells that lack a membrane-bound nucleus and membrane-bound cytoplasmic organelles. Their genetic material is present mainly in a region called the nucleoid.

Major characteristics

• Usually small in size.
• No true membrane-bound nucleus.
• DNA is generally circular and lies in the nucleoid.
• Membrane-bound organelles are absent.
• 70S ribosomes are present.
• Cell wall is present in most bacteria.
• Binary fission is the common mode of reproduction.
• Plasma membrane is present.

Eukaryotic Cell

Eukaryotic cells possess a well-defined nucleus enclosed by a nuclear envelope. They also contain several membrane-bound organelles that perform specialized functions.

Major characteristics

• Generally larger and structurally more complex.
• True membrane-bound nucleus is present.
• Membrane-bound organelles are present.
• Chromosomal DNA is mainly linear.
• 80S ribosomes occur in the cytoplasm.
• Mitochondria, ER, Golgi complex and lysosomes are present.
• Cell division occurs by mitosis and meiosis.

Prokaryotic and Eukaryotic Cells: Comparison

Cell Membrane

The cell membrane, also called the plasma membrane, is the selectively permeable boundary surrounding the cytoplasm. It separates the intracellular environment from the extracellular environment and regulates movement of substances into and out of the cell.

The widely accepted structural model is the fluid mosaic model, proposed by Singer and Nicolson in 1972.

Molecular Organization of the Plasma Membrane

1. Lipids

Phospholipids form the basic framework of the membrane. Each phospholipid contains a hydrophilic head and hydrophobic fatty-acid tails.

2. Proteins

Membrane proteins may be integral or peripheral. They function in transport, receptors, enzymatic reactions, cell recognition and communication.

3. Carbohydrates

Carbohydrates are usually attached to membrane proteins or lipids forming glycoproteins and glycolipids. Together they contribute to the glycocalyx.

4. Cholesterol

Cholesterol occurs between phospholipids in animal cell membranes. It contributes to membrane stability and helps regulate membrane fluidity.

Membrane Transport

Movement of substances across the plasma membrane is necessary for maintaining cellular homeostasis. Transport may occur without energy expenditure or may require metabolic energy.

A. Passive Transport

Passive transport occurs down an electrochemical or concentration gradient and does not directly require ATP.

• Simple diffusion: Movement of molecules from higher to lower concentration.
• Facilitated diffusion: Movement through specific membrane proteins.
• Osmosis: Movement of water across a selectively permeable membrane.

B. Active Transport

Active transport moves substances against their concentration or electrochemical gradient and requires energy.

A major example is the sodium-potassium pump (Na⁺/K⁺-ATPase), which uses ATP to transport sodium and potassium ions.

C. Bulk Transport

Large particles and macromolecules can be transported through vesicular mechanisms.

• Endocytosis: entry of material into the cell.
• Phagocytosis: uptake of large solid particles.
• Pinocytosis: uptake of extracellular fluid.
• Exocytosis: release of substances from the cell.

Introduction

The cytoskeleton is a dynamic network of protein filaments distributed throughout the cytoplasm. It provides structural support, maintains cell shape, assists intracellular transport and participates in cell movement and division.

Major Components

1. Microtubules

Microtubules are hollow cylindrical structures composed mainly of tubulin subunits. They are important in maintaining cell shape, intracellular transport, chromosome movement and the structure of cilia and flagella.

Functions

• Formation of mitotic spindle.
• Chromosome movement during cell division.
• Intracellular transport.
• Formation of cilia and flagella.
• Maintenance of cellular organization.

2. Microfilaments

Microfilaments are thin filaments composed mainly of actin. They are involved in cell movement, muscle contraction, cytokinesis and maintenance of cell shape.

3. Intermediate Filaments

Intermediate filaments are strong cytoskeletal fibres that provide mechanical strength and structural stability. Examples include keratin filaments and neurofilaments.

Cilia and Flagella

Cilia and flagella are specialized cell-surface structures involved in movement. In eukaryotes, their core is formed by a microtubular axoneme.

A typical motile eukaryotic cilium or flagellum has the 9 + 2 arrangement: nine peripheral microtubule doublets surrounding two central microtubules.

Comparison of Cytoskeletal Elements

Structure

The endoplasmic reticulum (ER) is an extensive membrane-bound network of flattened sacs and tubules distributed throughout the cytoplasm. It is continuous with the outer nuclear membrane.

Based on the presence or absence of ribosomes, ER is classified into rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).

Rough Endoplasmic Reticulum

RER has ribosomes attached to its cytoplasmic surface and is especially abundant in cells actively synthesizing proteins for secretion or membranes.

Functions of RER

• Protein synthesis.
• Initial protein folding.
• Modification of newly synthesized proteins.
• Transport of proteins toward the Golgi complex.

Smooth Endoplasmic Reticulum

SER lacks surface-bound ribosomes and participates in lipid synthesis, detoxification, carbohydrate metabolism and calcium storage in specialized cells.

Structure

The Golgi complex consists of stacks of flattened membrane-bound sacs called cisternae. It has distinct functional regions including cis, medial and trans compartments.

Functions

• Modification of proteins and lipids.
• Sorting and packaging of cellular products.
• Formation of secretory vesicles.
• Formation and processing of lysosomal enzymes.
• Participation in membrane trafficking.

Structure

Lysosomes are membrane-bound organelles containing hydrolytic enzymes capable of digesting proteins, lipids, carbohydrates and nucleic acids.

Functions

• Intracellular digestion.
• Digestion of foreign particles.
• Destruction of worn-out organelles through autophagy.
• Participation in cellular recycling.
• Removal of cellular debris.

Structure and Function

Peroxisomes are small membrane-bound organelles containing oxidative enzymes. They are involved in oxidation of fatty acids and detoxification of potentially harmful compounds.

Important enzymes

• Oxidases.
• Catalase.

Oxidases may generate hydrogen peroxide during oxidation reactions. Catalase converts hydrogen peroxide into water and oxygen, helping protect the cell from oxidative damage.

Introduction

Mitochondria are double-membrane-bound organelles responsible for most aerobic ATP production in eukaryotic cells. They are particularly abundant in cells with high energy requirements.

Structure

• Outer membrane: relatively permeable to small molecules.
• Intermembrane space: region between outer and inner membranes.
• Inner membrane: contains the electron transport chain and ATP synthase.
• Cristae: folds of the inner membrane that increase surface area.
• Matrix: contains enzymes of the citric acid cycle, mitochondrial DNA and ribosomes.

Functions

• ATP production through oxidative phosphorylation.
• Citric acid cycle reactions.
• Participation in programmed cell death.
• Contribution to cellular metabolism.

Structure

Ribosomes are non-membranous ribonucleoprotein particles composed of ribosomal RNA and proteins. They are the major sites of protein synthesis.

Types

• 70S ribosomes: characteristic of prokaryotes and also found in mitochondria and chloroplasts.
• 80S ribosomes: found mainly in the cytoplasm of eukaryotic cells.

Function

Ribosomes translate the nucleotide sequence of mRNA into a polypeptide chain during protein synthesis.

Introduction

The nucleus is the major genetic control centre of a eukaryotic cell. It contains most of the cell's genomic DNA and regulates gene expression, DNA replication and cell division.

Major Components

1. Nuclear Envelope

The nuclear envelope consists of two membranes separated by the perinuclear space. Nuclear pores provide regulated exchange between nucleus and cytoplasm.

2. Nucleoplasm

Nucleoplasm is the semifluid internal material of the nucleus containing chromatin, nucleolus and various nuclear proteins.

3. Chromatin

Chromatin consists mainly of DNA associated with histone and non-histone proteins. It is organized into higher-order structures within the nucleus.

4. Nucleolus

The nucleolus is a dense nuclear region involved mainly in ribosomal RNA synthesis and ribosomal subunit assembly.

Nucleocytoplasmic Interrelationship

The nucleus and cytoplasm function as an integrated system. DNA within the nucleus directs synthesis of RNA, while the cytoplasm contains ribosomes and many proteins required for cellular activity.

Messenger RNA is synthesized in the nucleus and transported through nuclear pores into the cytoplasm, where it is translated into protein. Conversely, many proteins synthesized in the cytoplasm are transported into the nucleus to participate in DNA replication, transcription and chromatin organization.

Chromosome

A chromosome is a highly organized DNA-protein structure that carries genetic information. During cell division, chromosomes become highly condensed and can be observed microscopically.

Chromosome Nomenclature

Chromosomes are commonly described according to the position of the centromere.

• Metacentric: centromere approximately central.
• Submetacentric: centromere slightly off-centre.
• Acrocentric: centromere close to one end.
• Telocentric: centromere terminal.

Karyotype

A karyotype is the complete chromosome complement of an organism arranged according to number, size, shape and centromere position.

In humans, somatic cells normally contain 46 chromosomes arranged as 23 pairs: 22 pairs of autosomes and one pair of sex chromosomes.

Giant Chromosomes

Polytene Chromosomes

Polytene chromosomes are giant chromosomes formed by repeated rounds of DNA replication without corresponding cell division. They are classically observed in the salivary glands of certain dipteran larvae.

Their characteristic banding pattern makes them useful for cytogenetic and gene-expression studies.

Lampbrush Chromosomes

Lampbrush chromosomes are exceptionally large meiotic chromosomes observed during the diplotene stage of oogenesis in many animals. They contain extended lateral loops associated with active transcription.

Euchromatin and Heterochromatin

Cell Cycle

The cell cycle is the ordered sequence of events through which a cell grows, duplicates its genetic material and divides.

Major phases

• G₁ phase: cell growth and synthesis of cellular components.
• S phase: DNA replication.
• G₂ phase: preparation for mitosis.
• M phase: nuclear division and cytokinesis.

Mitosis

Mitosis is an equational nuclear division in which one parent cell produces two genetically similar daughter nuclei, with chromosome number generally maintained.

Stages of Mitosis

1. Prophase: chromosomes condense and spindle formation begins.
2. Prometaphase: nuclear envelope breaks down and spindle microtubules interact with chromosomes.
3. Metaphase: chromosomes align at the metaphase plate.
4. Anaphase: sister chromatids separate and move toward opposite poles.
5. Telophase: daughter nuclei reform around chromosome sets.
6. Cytokinesis: cytoplasm divides to form daughter cells.

Meiosis

Meiosis is a specialized type of cell division that reduces the chromosome number by half and generates genetically diverse haploid cells.

It consists of two successive nuclear divisions: meiosis I and meiosis II, following one round of DNA replication.

Important events

• Synapsis of homologous chromosomes.
• Formation of bivalents and tetrads.
• Crossing over during prophase I.
• Separation of homologous chromosomes in meiosis I.
• Separation of sister chromatids in meiosis II.
• Formation of haploid daughter cells.

Mendelian Inheritance

Mendelian inheritance refers to inheritance patterns described from the experiments of Gregor Mendel using pea plants. Mendel established fundamental principles explaining how hereditary factors are transmitted from parents to offspring.

Law of Dominance

When two contrasting alleles occur together in a heterozygote, one allele may express itself phenotypically while the other remains unexpressed under complete dominance.

Law of Segregation

The two alleles of a gene separate during gamete formation so that each gamete normally receives one allele.

Independent Assortment

During gamete formation, alleles of different genes assort independently when the genes are sufficiently unlinked and located on different chromosomes or far apart on the same chromosome.

Non-Mendelian Inheritance

Not all inheritance patterns follow simple Mendelian dominance. Important examples include incomplete dominance, codominance, multiple alleles, polygenic inheritance, epistasis, mitochondrial inheritance and sex-linked inheritance.

Gene

A gene is a functional unit of heredity represented by a DNA sequence that contributes to a functional product such as an RNA molecule or protein.

Alleles

Alleles are alternative forms of a gene occupying corresponding positions at homologous chromosome loci.

Dominant Allele

A dominant allele is expressed in the phenotype of a heterozygote under a complete-dominance relationship.

Recessive Allele

A recessive allele is generally expressed phenotypically only when the relevant locus is homozygous under complete dominance.

Genotype and Phenotype

Multiple Alleles

Multiple alleles occur when a gene exists in more than two alternative forms within a population, although a diploid individual normally carries only two alleles at a particular locus.

ABO Blood Group System

The ABO blood group system is determined by the alleles Iᴬ, Iᴮ and i.

Iᴬ and Iᴮ are codominant with each other, while both are dominant over i.

Rh Blood Group

The Rh blood group system includes several antigens, of which the D antigen is particularly important clinically. Individuals possessing the D antigen are generally described as Rh-positive, while those lacking it are Rh-negative.

Rh incompatibility between an Rh-negative mother and an Rh-positive fetus can lead to maternal sensitization and, in subsequent pregnancies, hemolytic disease of the fetus and newborn under relevant circumstances.

Linkage

Linkage refers to the tendency of genes located on the same chromosome to be inherited together rather than assorting independently.

Genes located close together generally show stronger linkage because crossing over between them is less frequent.

Types

• Complete linkage: linked genes are inherited together with little or no detectable recombination.
• Incomplete linkage: some recombinant offspring occur because crossing over takes place between linked genes.

Crossing Over

Crossing over is the reciprocal exchange of corresponding segments between non-sister chromatids of homologous chromosomes during prophase I of meiosis.

The visible point of exchange is associated with a structure called a chiasma.

Significance of Crossing Over

• Produces new combinations of alleles.
• Increases genetic variation.
• Provides the basis for genetic mapping.
• Helps explain recombinant offspring.
• Contributes to evolutionary diversity.

Introduction

Sex-linked inheritance refers to inheritance of genes located on sex chromosomes. In humans, most sex-linked genes are located on the X chromosome.

X-Linked Inheritance

Males have one X chromosome and one Y chromosome, whereas females generally have two X chromosomes. Therefore, an X-linked allele in a male is usually expressed because there is no second X-linked allele at the same locus.

Characteristics of X-linked recessive inheritance

• More frequently expressed in males.
• No direct father-to-son transmission of an X-linked gene.
• An affected male passes his X chromosome to all daughters.
• A carrier female may transmit the allele to sons and daughters.

Examples

• Red-green colour vision deficiency.
• Haemophilia A.
• Haemophilia B.
• Duchenne muscular dystrophy.

Y-Linked Inheritance

Genes located on the Y chromosome are transmitted from an affected father to his sons. Such traits are therefore restricted to males in typical Y-linked inheritance.

X-Y Linked Genes

Genes located in the homologous regions shared by X and Y chromosomes are present on both sex chromosomes. These regions are called pseudoautosomal regions.

Non-disjunction as Evidence for Chromosomal Basis of Heredity

Non-disjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. It can produce gametes with abnormal chromosome numbers.

The classical experimental association between non-disjunction and sex-chromosome inheritance provided important cytological evidence supporting the chromosome theory of heredity.

Introduction

Sex determination is the biological mechanism by which an organism develops as male, female or, in some species, another sexual phenotype.

Major mechanisms

• XX–XY system: females are XX and males are XY, as in humans.
• XX–XO system: females are XX while males have a single X chromosome.
• ZZ–ZW system: males are ZZ and females are ZW, as in birds.
• Haplodiploidy: sex is associated with ploidy, as in many hymenopterans.
• Environmental sex determination: environmental conditions influence sexual development in some animals.

Chromosomal Aberration

Chromosomal aberrations are structural or numerical alterations involving chromosomes.

Numerical Changes

Numerical chromosome abnormalities can be broadly divided into euploidy and aneuploidy.

Euploidy

Euploidy involves changes in complete chromosome sets.

• Monoploidy: one complete chromosome set.
• Polyploidy: three or more complete chromosome sets.

Aneuploidy

Aneuploidy involves gain or loss of individual chromosomes rather than an entire chromosome set.

• Nullisomy: 2n − 2
• Monosomy: 2n − 1
• Trisomy: 2n + 1
• Tetrasomy: 2n + 2
• Double trisomy: gain of one chromosome in each of two different chromosome pairs.

Mutation

A mutation is a heritable alteration in genetic material. Mutations may occur spontaneously or result from exposure to mutagenic agents.

Types of Mutations

• Point mutation: alteration involving a single nucleotide pair.
• Substitution: one nucleotide is replaced by another.
• Insertion: addition of nucleotide(s).
• Deletion: loss of nucleotide(s).
• Frameshift mutation: insertion or deletion that changes the reading frame.

Pedigree Analysis

A pedigree is a graphical representation of the occurrence and transmission of a particular trait through several generations of a family.

Common symbols

• Square = male.
• Circle = female.
• Shaded symbol = affected individual.
• Horizontal line = mating relationship.
• Vertical line = descent.

Human Traits

Human traits may be influenced by single genes, multiple genes, environmental factors or combinations of genetic and environmental influences.

Examples of genetically influenced human characteristics include certain blood-group phenotypes, inherited metabolic traits and susceptibility to particular genetic disorders.

Sex-Linked Diseases

Several human genetic disorders follow sex-linked inheritance. Important examples include haemophilia A, haemophilia B, red-green colour vision deficiency and Duchenne muscular dystrophy.

Disorders Due to Mutant Genes

Mutations affecting specific genes may produce inherited disorders by altering the structure or function of a protein, enzyme or regulatory molecule.

Examples include sickle-cell disease, phenylketonuria, cystic fibrosis and certain forms of haemophilia.

Eugenics

Eugenics is a historical movement and set of ideas concerned with attempting to influence the genetic composition of human populations through selective reproduction. Its history includes serious ethical abuses and discriminatory practices.

Euphenics

Euphenics refers to attempts to improve the phenotype or manage the effects of genetic disorders by modifying environmental, physiological or biochemical conditions rather than changing the genotype itself.

Introduction

Genetic engineering refers to the deliberate modification, isolation, insertion or manipulation of genetic material using molecular biological techniques.

Basic Concept

A gene of interest can be isolated, inserted into a suitable vector and introduced into a host cell where it may be replicated or expressed.

Important Tools

• Restriction endonucleases: recognize specific DNA sequences and cleave DNA.
• DNA ligase: joins DNA fragments.
• Vectors: carry DNA fragments into host cells.
• DNA polymerases: synthesize DNA during amplification and replication-related procedures.

Applications of Genetic Engineering

• Production of recombinant proteins.
• Production of some therapeutic proteins.
• Development of genetically modified organisms.
• Research into gene function.
• Development of molecular diagnostic methods.
• Agricultural biotechnology.
• Pharmaceutical biotechnology.

Definition

Gene therapy is an approach in which genetic material is introduced, removed, altered or regulated in cells with the objective of treating or managing disease.

Basic Principle

If disease results from loss or abnormal function of a gene, therapeutic strategies may attempt to restore a functional genetic product, modify gene expression or otherwise alter the disease-associated genetic pathway.

Major Approaches

• Gene addition: introducing a functional gene or genetic sequence.
• Gene editing: targeted modification of a DNA sequence.
• Gene silencing: reducing expression of a harmful or abnormal gene product.

Applications

• Inherited genetic disorders.
• Some cancers.
• Selected immune disorders.
• Research into rare genetic diseases.

Quick Revision – Cell Biology

Quick Revision – Genetics