Genome: A Complete Quick Revision Guide

 

Genome: A Complete Quick Revision Guide

Introduction

The genome is the complete set of hereditary material present in an organism. It contains all the genetic instructions necessary for growth, development, reproduction, metabolism, adaptation and evolution. In most living organisms, the genome consists of DNA, while in some viruses it consists of RNA. The study of genomes is known as genomics, one of the most rapidly developing branches of biology. It has revolutionized medicine, agriculture, biotechnology, forensic science and evolutionary biology.

What Is a Genome?

A genome is the complete genetic material present in the haploid set of chromosomes of an organism. The term Genome was introduced by Hans Winkler in 1920.

It includes:

• Protein-coding genes
• Regulatory DNA
• Introns
• Exons
• Non-coding RNAs
• Repetitive DNA
• Structural DNA sequences

GENOME VS GENE

Gene	Genome
A segment of DNA	Entire hereditary material
Codes for one functional RNA or protein	Contains all genes and non-coding DNA
Functional unit of heredity	Complete genetic blueprint

Types of Genomes

Living organisms possess different types of genomes.

Nuclear Genome

• Present inside the nucleus.
• Largest genome in eukaryotes.
• Contains most genes.

Mitochondrial Genome

• Circular DNA.
• Inherited maternally in humans.
• Contains 37 genes.
• Size: 16,569 base pairs.

Chloroplast Genome

• Present in plants and algae.
• Responsible for photosynthetic functions.

Viral Genome

May consist of:

• DNA
  or
• RNA

Prokaryotic VS Eukaryotic Genome

Feature	Prokaryotes	Eukaryotes
Chromosomes	Usually one	Multiple
Shape	Circular	Linear
Histones	Absent in bacteria	Present
Introns	Rare	Common
Gene Density	High	Lower
Repetitive DNA	Minimal	Abundant

Coding and Non-Coding DNA

The human genome contains both coding and non-coding DNA.

Coding DNA

Produces proteins. Accounts for only about 1–2% of the genome.

Non-coding DNA

Includes:

• Introns
• Regulatory sequences
• Telomeres
• Centromeres
• Repetitive DNA
• Non-coding RNA genes

Although it does not encode proteins, it performs essential regulatory and structural functions.

Genome Size

Genome size is measured in:

• Base pairs (bp)
• Kilobases (kb)
• Megabases (Mb)
• Gigabases (Gb)

Human Genome

Approximately: 3.2 billion base pairs (3.2 Gb)

C-Value

The C-value is the amount of DNA present in the haploid nucleus of an organism.

C-value paradox

The C-value paradox states that genome size does not necessarily correlate with the complexity of an organism because much of the DNA is non-coding or repetitive.

Genome Organization

DNA is highly organized to fit inside the nucleus. Packaging levels are:

DNA

 ↓

Nucleosome

 ↓

Chromatin Fiber

 ↓

Looped Domains

 ↓

Chromosome

Nucleosome

The nucleosome is the basic structural unit of chromatin. It consists of:

• Approximately 146 base pairs of DNA
• Wrapped around a histone octamer:
• 2 × H2A
• 2 × H2B
• 2 × H3
• 2 × H4

Histone H1 binds linker DNA.

Euchromatin and Heterochromatin

Euchromatin

• Loosely packed
• Gene-rich
• Transcriptionally active
• Lightly stained

Heterochromatin

• Highly condensed
• Gene-poor
• Transcriptionally inactive or less active
• Darkly stained

Exons and Introns

Exons

Remain in mature mRNA. Usually code for proteins.

Introns

Removed during RNA splicing. Help regulate gene expression and allow alternative splicing.

Repetitive DNA

Major types include:

• Satellite DNA
• Minisatellites
• Microsatellites (STRs)

Functions:

• Chromosome stability
• Gene regulation
• DNA fingerprinting
• Genome evolution

Telomeres

Specialized DNA sequences present at chromosome ends. Human telomeric repeat:

TTAGGG

Functions:

• Protect chromosome ends.
• Prevent chromosome fusion.
• Maintain chromosome stability.

Centromere

The centromere is the primary constriction where spindle fibres attach through the kinetochore during cell division.

The Human Genome

The human genome consists of:

• 23 chromosomes (haploid)
• 46 chromosomes (diploid)

Genome size: 3.2 Gb

Protein-coding genes: Approximately 19,000–21,000 (NCERT historically mentions 20,000–25,000). DNA in one diploid cell measures approximately 2 metres when fully stretched.

Human Genome Composition

Approximate composition:

Component	Percentage
Protein-coding DNA	1–2%
Non-coding DNA	>98%

The non-coding fraction includes regulatory DNA, introns, repetitive DNA and structural elements.

Genetic Variation

Although all humans are remarkably similar, approximately 99.9% of DNA sequence is identical between unrelated individuals. Variation arises from:

• Mutations
• Recombination
• Independent assortment
• Random fertilization

SNPs

Single Nucleotide Polymorphisms (SNPs) are the most common type of genetic variation.

Applications:

• Disease prediction
• Pharmacogenomics
• Population genetics

Copy Number Variations (CNVs)

CNVs involve deletion or duplication of DNA segments. They influence:

• Gene dosage
• Disease susceptibility
• Individual variation

The Human Genome Project (HGP)

The Human Genome Project was one of the largest international scientific collaborations.

Timeline

Event	Year
Started	1990
Draft Genome	2000
Completed	2003

Duration: 13 years

Objectives of the HGP

• Identify all human genes.
• Sequence the complete human genome.
• Develop genomic databases.
• Improve sequencing technology.
• Develop bioinformatics tools.
• Address ethical, legal and social issues.

Methods Used

Major steps:

1. DNA isolation
2. Fragmentation
3. Cloning into BAC vectors
4. Physical mapping
5. DNA sequencing
6. Computer assembly
7. Genome annotation

The principal sequencing method was Sanger chain-termination sequencing.

Bioinformatics

Bioinformatics combines biology, computer science and statistics to analyze genomic data. Applications include:

• Sequence analysis
• Genome assembly
• Gene prediction
• Comparative genomics
• Drug discovery

Major Findings of the HGP

• Human genome contains approximately 3.2 billion base pairs.
• Humans possess approximately 20,000–25,000 genes (NCERT; current annotation identifies about 19,000–21,000 protein-coding genes).
• Only 1–2% of the genome directly encodes proteins.
• Approximately 99.9% of human DNA is identical among unrelated individuals.
• A large proportion of the genome consists of repetitive and non-coding DNA.

Modern Genomics

The Human Genome Project gave rise to several specialized branches.

Comparative Genomics

Compares genomes of different species to study evolution and gene function.

Functional Genomics

Studies gene function and gene regulation.

Structural Genomics

Studies genome organization and protein structures.

Transcriptomics

Studies the complete RNA population (transcriptome).

Proteomics

Studies the complete protein complement (proteome).

Metabolomics

Studies all metabolites produced by cells.

Epigenomics

Studies heritable changes in gene expression without changing the DNA sequence. Major mechanisms are:

• DNA methylation
• Histone modification
• Non-coding RNAs

Pharmacogenomics

Studies how genes influence responses to medicines. Used in personalized drug therapy.

Precision Medicine

Uses genomic information to customize healthcare according to an individual's genetic makeup.

CRISPR-Cas9

CRISPR-Cas9 is the most widely used genome-editing technology.

Functions:

• Deletes genes
• Inserts genes
• Repairs mutations
• Modifies DNA precisely

Applications:

• Gene therapy
• Cancer research
• Crop improvement
• Biotechnology

Applications of Genomics

Medicine

• Genetic diagnosis
• Prenatal testing
• Cancer genomics
• Precision medicine
• Gene therapy

Agriculture

• Disease-resistant crops
• High-yield varieties
• Stress-tolerant plants

Forensic Science

• DNA fingerprinting
• Paternity testing
• Criminal investigations

Evolutionary Biology

• Human origins
• Comparative genomics
• Population genetics

HIGH-YIELD NEET FACTS

✔ Genome = Complete hereditary material.

✔ Coined by Hans Winkler (1920).

✔ Human genome = 3.2 Gb.

✔ Human haploid chromosomes = 23.

✔ Human diploid chromosomes = 46.

✔ Protein-coding DNA = 1–2%.

✔ Human mitochondrial genome = 16,569 bp; 37 genes.

✔ Nucleosome = 146 bp DNA + histone octamer.

✔ Human telomeric sequence = TTAGGG.

✔ Human Genome Project = 1990–2003.

✔ Human DNA similarity = 99.9%.

✔ Most common genetic variation = SNP.

✔ Genome editing = CRISPR-Cas9.

✔ DNA fingerprinting mainly uses VNTRs/STRs.

MEMORY MAP

GENOME

│

├── Nuclear Genome

├── Mitochondrial Genome

├── Coding DNA

├── Non-coding DNA

│

├── Genome Organisation

│      ├── Chromatin

│      ├── Nucleosome

│      ├── Euchromatin

│      ├── Heterochromatin

│

├── Human Genome

│      ├── 3.2 Gb

│      ├── 23 Chromosomes

│      ├── 20,000–25,000 genes (NCERT)

│

├── Human Genome Project

│      ├── 1990–2003

│      ├── Sequencing

│      ├── Bioinformatics

│

└── Modern Genomics

       ├── Comparative

       ├── Functional

       ├── Transcriptomics

       ├── Proteomics

       ├── Epigenomics

       ├── Pharmacogenomics

       └── CRISPR

One-Page Final Revision

Topic	Key Fact
Genome	Complete hereditary material
Coined by	Hans Winkler (1920)
Human Genome	~3.2 Gb
Haploid Chromosomes	23
Diploid Chromosomes	46
Protein-Coding DNA	1–2%
Human Protein-Coding Genes	~19,000–21,000 (NCERT: 20,000–25,000)
Mitochondrial Genome	16,569 bp, 37 genes
Nucleosome	~146 bp DNA + histone octamer
Euchromatin	Active chromatin
Heterochromatin	Condensed, less active chromatin
Telomeric Repeat	TTAGGG
HGP Duration	1990–2003
Human DNA Similarity	~99.9%
Most Common Variation	SNP
Genome Editing	CRISPR-Cas9
DNA Fingerprinting	VNTRs and STRs