DNA Structure and Replication: Study Notes

DNA Structure and Replication

Nucleic Acids

Nucleic acids are the macromolecules responsible for storing and transmitting genetic information in living organisms. The two main types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• Genes are sections of DNA that serve as blueprints for making proteins, located at specific points on chromosomes.

• Proteins carry out all cellular activities, and their synthesis is directed by genes.

• DNA and RNA differ in structure and function, but both are essential for genetic processes.

Nucleotides

Nucleotides are the monomers that make up nucleic acids. Each nucleotide consists of three components:

• Sugar: Deoxyribose in DNA, ribose in RNA

• Phosphate group

• Nitrogen base:

  • Adenine (A)

  • Guanine (G)

  • Cytosine (C)

  • Thymine (T, DNA only)

  • Uracil (U, RNA only)

DNA Structure

DNA is characterized by its double helix structure, resembling a twisted ladder.

• Sugar-phosphate backbone: Formed by strong covalent bonds between sugars and phosphates.

• Nitrogen bases: Bond in the middle with weak hydrogen bonds.

• Complementary base pairing rules:

  • Adenine (A) pairs with Thymine (T)

  • Cytosine (C) pairs with Guanine (G)

RNA Structure

RNA is typically single-stranded and contains exposed bases.

• RNA bases can bind with DNA bases during processes such as transcription.

• Base pairing in RNA:

  • Adenine (A) pairs with Uracil (U)

  • Cytosine (C) pairs with Guanine (G)

DNA vs. RNA

The following table summarizes the key differences between DNA and RNA:

	DNA	RNA
Types of nitrogen bases	A, T, C, G	A, U, C, G
Type of sugar used	deoxyribose	ribose
Shape	double helix	single strand

Basics of Heredity

Genetic information is organized into chromosomes and genes.

• Chromosomes: Tightly coiled strands of DNA; different organisms have different numbers of chromosomes.

  • Humans: 23 pairs (46 total)

  • Dogs: 37 pairs (74 total)

• Gene: A section of DNA with instructions to code for a protein; one chromosome can contain thousands of genes.

• Genes are pieces of DNA; chromosomes are long strands of DNA bundled together.

DNA Replication

DNA replication is the process by which a cell makes an identical copy of its DNA before cell division.

• Occurs in the nucleus during the S Phase (Synthesis) of Interphase.

• Ensures that each new cell has the same DNA as the original cell.

Steps of DNA Replication

1. Unzip the DNA double helix.

2. Enzymes (e.g., DNA polymerase) find complementary bases and bind them according to base-pairing rules (A-T and C-G).

3. Two identical DNA molecules are formed, each with one "old" strand and one "new" strand.

Semi-Conservative Replication

• DNA replication is semi-conservative: each new DNA molecule consists of one original (parent) strand and one newly synthesized strand.

• The parent strand serves as a template for the new strand.

• This mechanism preserves genetic information across generations.

Considerations and Implications of Replication Errors

Errors can occur during DNA replication, leading to mutations.

• Types of mistakes: Incorrect base pairing, insertions, deletions, or duplications.

• Implications:

  • Errors in body (somatic) cells may affect only the individual.

  • Errors in germ cells (sperm or egg) can be passed to offspring.

  • Mutations in a gene may affect a single protein; mutations in a chromosome may impact many genes.

Example: A mutation in the gene coding for hemoglobin can cause sickle cell anemia, while a chromosomal mutation may result in conditions such as Down syndrome. Additional info: DNA replication fidelity is maintained by proofreading enzymes, but some errors escape correction, contributing to genetic diversity and disease.