Molecular Structure of Genes and Chromosomes
Definition & Basics
Gene: A segment of DNA that carries instructions for synthesizing a protein or functional RNA (e.g., rRNA, tRNA).
Example: The HBB gene codes for the beta-globin subunit of hemoglobin.
Chromosome: A highly condensed structure of DNA + proteins (histones) that stores genetic information.
Humans have 46 chromosomes (23 pairs) in somatic cells.
Molecular Structure
A. DNA Structure
Two antiparallel polynucleotide chains linked by hydrogen bonds between complementary bases:
Adenine (A) ⇄ Thymine (T)
Guanine (G) ⇄ Cytosine (C)
Backbone: Sugar (deoxyribose) + phosphate.
Key Terms:
Nucleotide: Sugar + phosphate + nitrogenous base.
Gene Locus: Specific location of a gene on a chromosome.
B. Chromosome Structure
DNA Packaging: From DNA to Chromosomes
To fit the vast length of DNA within the nucleus, it must be highly condensed and organized into chromosomes. This is achieved through a multi-level packaging process involving proteins called histones.
Histones: Positively charged proteins that DNA wraps around. The main types are H2A, H2B, H3, and H4.
Nucleosomes: The basic structural unit of chromatin. DNA wraps around a core of eight histone proteins (two of each type: H2A, H2B, H3, H4), forming a "bead-like" structure. Linker DNA connects nucleosomes.
Chromatin: The complex of DNA and proteins (including histones) that makes up chromosomes. Chromatin exists in different levels of organization:
10 nm Fiber ("Beads on a String"): Nucleosomes connected by linker DNA. This is the most basic level of organization.
30 nm Fiber: The nucleosomes are then wrapped into a 30 nm spiral called a solenoid. Histone H1 plays a crucial role in this level of compaction.
Looped Domains: The 30 nm fiber is organized into loops attached to a protein scaffold.
Higher-Level Condensation: Further folding and coiling of looped domains during cell division (mitosis and meiosis) leads to highly condensed, visible chromosomes.
Chromatin is a complex of DNA and proteins that condenses DNA into chromosomes. It's found in the nucleus of eukaryotic cells, such as those of humans and other higher organisms.
How chromatin works
Chromatin is made up of repeating units called nucleosomes.
Each nucleosome is made up of DNA wrapped around eight histone proteins.
Chromatin's role
Chromatin's primary function is to package long DNA molecules into more compact, denser structures.
Chromatin also controls how the genome is read out from cell to cell.
Chromatin is incredibly dynamic, reorganizing during development and in response to environmental stimuli.
Chromatin and disease
Dysregulation of chromatin underlies a number of diseases including developmental disorders, cancer, heart disease, and neurological disorders.
Types of Chromatins:
Euchromatin: Loosely packed chromatin. Genes in euchromatin are actively transcribed (expressed). It is less condensed and allows access for enzymes involved in DNA replication and transcription.
Heterochromatin: Highly condensed chromatin. Genes in heterochromatin are generally transcriptionally inactive (not expressed). It is densely packed and less accessible.
· Chromosome Components:
o Centromere: The constricted region of a chromosome where sister chromatids are joined. It plays a vital role in chromosome segregation during cell division. Kinetochores, protein structures, assemble at the centromere and are the sites where microtubules attach to pull sister chromatids apart.
o Telomeres: Protective caps at the ends of chromosomes. They prevent chromosome ends from fraying or fusing with each other and maintain chromosome stability. Telomeres shorten with each cell division in most somatic cells.
o Chromosome Arms: The regions of the chromosome extending from the centromere to the telomeres. Shorter arm is designated as 'p' arm and longer arm is designated as 'q' arm.
· Types of Chromosomes (Based on Centromere Position):
o Metacentric: Centromere is located in the middle, resulting in two arms of equal length.
o Submetacentric: Centromere is slightly off-center, resulting in arms of unequal length.
o Acrocentric: Centromere is located very close to one end, resulting in one very short arm and one long arm.
o Telocentric: Centromere is located at the very end of the chromosome. (Humans do not have telocentric chromosomes).
IV. Relationship Between Genes and Chromosomes:
Chromosomes are the structures that carry genes. Think of chromosomes as chapters in a book, and genes as sentences within those chapters.
Each chromosome contains many genes arranged linearly along its DNA molecule.
The precise location of a gene on a chromosome is called its locus.
V. Key Differences and Similarities:
VI. Important Formulas & Rules
Chargaff’s Rules:
In DNA, A = T and G = C.
Ratio of A+T / G+C varies between species.
VII. Key Points & Mnemonics
Central Dogma: DNA → RNA → Protein.
Types of Chromosomes (based on centromere position):
Metacentric (middle), Submetacentric, Acrocentric, Telocentric (tip).
Mnemonic for DNA Bases:
"All Tigers Can Growl" → Adenine, Thymine, Cytosine, Guanine.
Common Mistakes:
Confusing chromatin (uncondensed DNA) with chromosomes (condensed).
Gene ≠ Allele: A gene is a location; an allele is a variant of the gene.
VIII. Previous Year Questions
Q: Which bond holds DNA strands together?
A: Hydrogen bonds between complementary bases.Q: What is the role of histones in chromosomes?
A: Package DNA into nucleosomes for compaction.Q: If a DNA strand has 30% Adenine, what is the % of Guanine?
A: A = 30%, so T = 30% → G + C = 40% → G = 20%.
Chargaff's rule:
According to Chargaff's rule, the amount of Adenine (A) is equal to Thymine (T) and the amount of Guanine (G) is equal to Cytosine (C).
Calculation:
If Adenine is 30%, then Thymine is also 30%, which means A + T = 60%. This leaves 40% for Guanine and Cytosine combined, so each would be 20%.
Key points:
A = T, G = C, and A + T + G + C = 100.