DNA and Chromosomes: Structure, Function, and Organization in Eukaryotic Cells

Ch.5. DNA and Chromosomes

Introduction

Genetic information is fundamental to life, governing reproduction and heredity. In eukaryotic cells, DNA is packaged into chromosomes within the nucleus. This guide explores the structure and function of DNA and chromosomes, emphasizing their roles in genetic information storage, transmission, and regulation.

The Structure and Function of DNA

Nucleic Acids and Nucleotides

• Nucleic acids include DNA and RNA, which are polymers of nucleotides.

• DNA stores genetic information; RNA translates this information into proteins.

• Recent discoveries show RNA also regulates gene expression via RNA interference through siRNA and microRNA.

Proteins vs DNA

• Proteins are composed of 20 amino acids, leading to complex and diverse functions.

• DNA is composed of 4 nucleotides, resulting in a structure that is simple and accurate for information storage.

DNA Sequence – The Genetic Language

• DNA uses four bases (A, T, G, C) to encode genetic information.

• The sequence of these bases forms the genetic code, which is consistent, stable, and accurate.

• Proteins use amino acids, but DNA’s four-letter code is optimal for genetic information storage.

DNA Structure

Structural Unit of DNA: Deoxynucleotides

• Each nucleotide consists of a phosphate group, deoxyribose sugar, and a nitrogenous base (A, T, G, C).

• Nucleotides are linked by phosphodiester bonds to form the DNA backbone.

DNA as a Polymer

• DNA is a linear polymer of deoxynucleotides.

• The backbone is formed by repeating units of phosphate-deoxyribose.

• DNA strands have directionality: 5' to 3'.

The Double Helix Model

• DNA exists as a double helix, discovered by Watson and Crick.

• Two complementary strands are antiparallel and wrap around each other.

• Bases are inside the helix, paired by hydrogen bonds (A-T, G-C).

• Base pairs are stabilized by hydrophobic interactions.

The Base Pair Rule

• Adenine (A) pairs with Thymine (T); Guanine (G) pairs with Cytosine (C).

• This base pairing ensures accurate replication and transmission of genetic information.

Example:

Strand 1	Strand 2 (Complementary)
5'-TGCAGTCGACTACATAG-3'	3'-ACGTCAGCTGATGTATC-5'

DNA Replication

• DNA replication is semiconservative: each new DNA molecule contains one original and one new strand.

• This mechanism ensures genetic continuity across generations.

Equation:

$\text{New DNA strand} = \text{Template strand} + \text{Complementary nucleotides}$

Organization of Eukaryotic Chromosomes

Chromosome Structure

• Each chromosome contains a single, highly condensed DNA molecule.

• Human DNA, if stretched out, would be about 2 meters long, but is packed into a nucleus about 6 μm in diameter.

• Chromosome components include telomeres, centromeres, and chromatids.

Chromosomes in Human Cells

• A karyotype is an ordered display of chromosome pairs.

• Humans have 23 pairs of chromosomes: 22 autosomes and 1 pair of sex chromosomes (XX for females, XY for males).

• Homologous chromosomes carry genes controlling the same inherited traits.

DNA Condensation in Chromosomes

• DNA is highly condensed in chromosomes, forming visible structures during cell division.

• Condensation is achieved through multiple levels of DNA packaging.

Chromosome Composition: DNA and Proteins

• Chromosomes are complexes of DNA and proteins called histones.

• Histones are basic proteins that help package DNA into structural units called nucleosomes.

Table: Chromosome Components

Component	Description
DNA	Genetic material, double helix
Histones	Basic proteins, form nucleosomes
Nucleosome	DNA wrapped around histone octamer
Chromatin	DNA-protein complex, visible in interphase

The Nucleosome and Chromatin Structure

Nucleosome Structure

• The nucleosome core consists of a disc-shaped histone octamer: H2A, H2B, H3, H4 (two copies each), plus H1.

• About 146 nucleotide pairs of DNA wrap around each nucleosome.

Chromatin vs Chromosomes

• Chromatin is the DNA-protein complex observed in non-dividing (interphase) cells.

• Chromosomes are highly condensed chromatin, visible during cell division.

Table: Chromatin vs Chromosome

Feature	Chromatin	Chromosome
State	Less condensed	Highly condensed
Visibility	Not visible under light microscope	Visible during mitosis/meiosis
Function	Gene expression, DNA replication	DNA segregation

Chromosome Dynamics and Cell Cycle

Chromosome Forms During Cell Cycle

• Chromosomes exist in different forms throughout the cell cycle.

• In dividing cells, DNA is highly packed in chromosomes for accurate segregation.

• In non-dividing cells, chromatin is less condensed, allowing gene expression and DNA replication.

Importance of DNA Packaging

• DNA packaging allows efficient storage and regulation of genetic information.

• Dynamic changes in chromatin structure regulate gene expression and cell division.

Central Dogma of Molecular Biology

Genetic Information Flow

• Genetic information is copied from DNA to RNA through transcription.

• RNA is translated into proteins, which perform cellular functions.

• Selective gene expression allows different cell types to arise from the same genetic material.

Equation:

$\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Summary Table: Key Features of DNA and Chromosomes

Feature	Description
Double Helix	Two antiparallel strands, base pairing
Base Pair Rule	A-T, G-C pairing ensures accuracy
Semiconservative Replication	Each new DNA has one old and one new strand
Chromatin	DNA-protein complex, regulates gene expression
Chromosome	Highly condensed DNA, visible during cell division
Nucleosome	Basic unit of DNA packaging

Additional info: RNA interference (siRNA, microRNA) is a key regulatory mechanism in eukaryotic gene expression, not covered in detail in the original notes but essential for understanding modern cell biology.