Ch. 5 Structure of DNA and Chromosomes

Structure of DNA

• Deoxyribonucleic Acid (DNA)

  • Definition: A double-stranded polynucleotide composed of two separate chains of covalently linked deoxynucleotides.

  • Function: DNA serves as the genetic information of cells, transmitting this information from generation to generation.

• Double Helix

  • Definition: The typical structure of a DNA molecule where two complementary strands are wound around each other.

  • Base-Pairing: The two strands are held together by base-pairing interactions between nucleotides.

  • Example Base Pairs:

  • Adenine (A) pairs with Thymine (T)

  • Cytosine (C) pairs with Guanine (G)

• Base Pair

  • Definition: Two complementary nucleotides in a DNA or RNA molecule that are held together by hydrogen bonds.

  • Examples of pairs:

  • A-T

  • G-C

• Gene Expression

  • Definition: The process through which a gene directs the synthesis of a protein or functional RNA molecule, producing a product that is useful to the cell or organism.

• Genetic Code

  • Definition: A set of rules that describes how the information contained in the nucleotide sequence of a gene is translated into the amino acid sequence of a protein.

Structure of Eukaryotic Chromosomes

• Chromosome

  • Definition: A long, thread-like structure made of DNA and proteins that carries genetic information in organisms.

  • Visibility: Chromosomes become visible when a plant or animal cell prepares to divide.

• Karyotype

  • Definition: An ordered display of the complete set of chromosomes of a cell, arranged by size and number.

• Gene

  • Definition: A unit of heredity containing instructions dictating the characteristics or phenotype of an organism.

  • Composition: A segment of DNA that directs the production of a particular protein or functional RNA molecule.

• Genome

  • Definition: The total genetic information carried by all chromosomes of a cell or organism.

Chromosome Components and Structure

• Replication Origin

  • Definition: The nucleotide sequence where DNA replication is initiated.

• Telomere

  • Definition: A repetitive nucleotide sequence that caps the ends of linear chromosomes to protect them.

  • Function: The telomere shortens with each round of replication.

• Centromere

  • Definition: A specialized DNA sequence that allows duplicated chromosomes to be separated during the M phase of the cell cycle.

• Histones

  • Definition: A small group of highly conserved proteins around which DNA wraps to form nucleosomes, representing the most fundamental level of chromatin packing.

Cell Cycle

• Phases of the Cell Cycle

  • G1 Phase: Growth phase before DNA synthesis.

  • S Phase: DNA replication.

  • G2 Phase: Preparation for mitosis.

  • M Phase: Cell division, which includes mitosis and cytokinesis.

• Mitosis Stages

  • Prophase

  • Metaphase

  • Anaphase

  • Telophase

Chromatin Structure

• Chromatin

  • Definition: A complex of DNA and proteins that make up the chromosomes in eukaryotic cells.

  • Composition: DNA is wrapped around histone proteins in a structure known as a nucleosome.

  • Nucleosome

  • Definition: A bead-like structural unit of a eukaryotic chromosome composed of a short length of DNA wrapped around an octameric core of histone proteins.

  • Composition includes nucleosomal core particles (DNA plus histone proteins) along with linker DNA that connects core particles.

• Chromatin Remodeling

  • Chromatin Remodeling Complex: A family of enzymes that utilize ATP hydrolysis to change the arrangement of nucleosomes, altering DNA accessibility for additional protein interactions.

• Histone-Modifying Enzymes

  • Definition: Enzymes that catalyze the covalent addition of small molecules (such as methyl groups or acetyl groups) to specific amino acids on histone proteins, influencing gene expression.

• Types of Chromatin

  • Euchromatin: Less compact, gene-rich areas of chromatin that allow access for transcription proteins.

  • Heterochromatin: Highly condensed, transcriptionally inactive regions of chromatin that are generally gene-poor.

DNA Backbone and Nucleotides

• DNA Backbone Structure

  • Composition: Alternating phosphate sugar structure (Phosphate-Sugar-Phosphate-Sugar).

  • Directionality: 5' (phosphate) → 3' (hydroxyl on the sugar).

  • Covalent Bonding: The sugar of one nucleotide is covalently bonded to the phosphate of the adjacent nucleotide.

• Nucleotide Composition

  • Components: Each nucleotide consists of a phosphate group, a sugar, and a nitrogenous base.

  • Examples of Bases:

  • Adenine (A)

  • Guanine (G)

  • Cytosine (C)

  • Thymine (T)

• Base Pairing

  • Hydrogen Bonds:

  • A-T pairs form 2 hydrogen bonds.

  • C-G pairs form 3 hydrogen bonds.

Organization of DNA

• DNA Condensation

  • DNA is organized into chromosomes through loops and coils around proteins, becoming approximately 10,000 times more compact than the double helix.

• Human Genome

  • Humans have a total of 46 chromosomes (23 pairs), each chromosome consisting of a single linear DNA molecule.

• Homologous Chromosomes

  • Definition: Chromosome pairs derived from each parent that are similar in length and gene content.

• Accessibility of DNA

  • Chromatin needs to be accessible for gene expression, replication, and repair.

  • Less condensed DNA is more easily accessible to transcription and replication machinery.