MBOC7_lectureppt_Ch04-A+F24

Chapter 4: DNA, Chromosomes, and Genomes

Introduction to Molecular Biology

• Textbook Reference: Molecular Biology of the Cell, Seventh Edition, by Alberts, Heald, Johnson, Morgan, Raff, Roberts, Walter.

• Copyright ©2022 W. W. Norton & Company, Inc.

Chapter Contents

• The Structure and Function of DNA

• Chromosomal DNA and its Packaging into the Chromatin Fiber

• The Effect of Chromatin Structure on DNA Function

• The Global Structure of Chromosomes

The Structure and Function of DNA

• Composition of DNA

  • DNA molecules consist of two complementary chains of nucleotides.

  • Directionality: DNA is directional (5' to 3' end).

• Denaturation of DNA

  • Can occur through breaking non-covalent bonds (hydrogen bonds) using pH and temperature changes.

  • Denatured DNA forms a single strand.

• Charge of DNA

  • Nucleotides are negatively charged due to phosphate groups.

  • The overall charge of DNA is negative.

Chromosomal DNA and Its Packaging into the Chromatin Fiber

• Eukaryotic DNA Organization

  • DNA is always associated with proteins (histones) forming chromatin.

  • Chromatin can be in a thin form or compacted state during mitosis.

• Basic Chromosome Structure

  • Includes centromeres, telomeres, and origins of replication.

  • Chromosomes contain long strings of genes; e.g., human genome contains about 20,000 protein-coding genes.

Nucleosomes and Chromatin Structure

• Definition and Structure of Nucleosomes

  • Nucleosomes are the basic unit of eukaryotic chromosome structure and include histones and DNA.

  • Consist of around 200 nucleotide pairs of DNA associated with a histone octamer.

• Dynamic Nature of Nucleosomes

  • Nucleosomes have a dynamic structure and are subject to changes by ATP-dependent chromatin-remodeling complexes.

  • Stabilization of nucleosomes through ionic and hydrogen bonding from histones.

The Global Structure of Chromosomes

• Chromosome Loops and Gene Expression

  • Chromosome loops compact the DNA for efficient storage.

  • Loops decondense during gene expression enabling RNA polymerase to access DNA.

• Chromosome Spatial Organization

  • Interphase chromosomes occupy discrete territories in the nucleus with specific locations for heterochromatin and euchromatin.

  • Euchromatin consists of actively expressed genes while heterochromatin might restrict gene expression.

Epigenetics and Gene Regulation

• Histone Modifications

  • Core histones undergo various covalent modifications affecting DNA packaging and gene expression.

  • For example, methylation often relates to gene silencing, while acetylation may lead to gene activation.

• Inheritance of Chromatin States

  • Some chromatin modifications can be inherited, influencing gene expression patterns in daughter cells.

Conclusion

• The structure and function of DNA are central to genetics and cellular functions.

• Understanding the dynamic processes of chromatin organization contributes greatly to the field of molecular biology.