DNA: The Molecule of Life

Gene and DNA Structure

Genes are the fundamental units of hereditary information, consisting of specific nucleotide sequences in DNA. DNA, or Deoxyribonucleic Acid, is the molecule that holds the instructions for all living things, primarily by coding for proteins.

• Gene: A discrete unit of hereditary information; a segment of DNA that codes for a specific protein.

• DNA Structure: Double helix composed of two strands.

• Nucleotide: The building block of DNA, consisting of a sugar (deoxyribose), a phosphate group, and a nitrogenous base.

• Bases: Four types—Adenine (A), Thymine (T), Guanine (G), Cytosine (C).

• Base Pairing: A pairs with T; G pairs with C.

Example: The sequence ATGC in DNA codes for a specific amino acid sequence in a protein.

DNA Replication

DNA replication is the process by which DNA makes a copy of itself before cell division. It is described as semi-conservative, meaning each new DNA molecule contains one original strand and one newly synthesized strand.

• Semi-Conservative Replication: Parent double helix splits; each strand serves as a template for a new strand.

Equation:

Central Dogma: DNA → RNA → Protein

The flow of genetic information in cells follows the central dogma: DNA is transcribed into RNA, which is then translated into protein.

• DNA: Stores genetic information.

• RNA: Transfers information and controls protein synthesis.

• Protein: Performs cellular functions.

RNA Structure and Function

RNA, or Ribonucleic Acid, is a single-stranded molecule involved in protein synthesis. Its sugar is ribose, and it uses Uracil (U) instead of Thymine.

• Single-stranded

• Bases: Adenine (A), Uracil (U), Guanine (G), Cytosine (C)

• Base Pairing: A pairs with U; G pairs with C

Protein Synthesis: Transcription and Translation

Protein synthesis involves two main processes: transcription and translation.

• Transcription:

  • DNA strand is copied into a complementary mRNA strand.

  • Occurs in the nucleus; product is mRNA.

• Translation:

  • mRNA is decoded into a polypeptide chain (protein) by tRNA.

  • tRNA brings matching amino acids for each codon; anti-codon on tRNA pairs with codon on mRNA.

  • Occurs in ribosomes; product is protein.

Equation:

Codons and Anti-codons

A codon is a three-base sequence on mRNA that codes for a specific amino acid. An anti-codon is a three-base sequence on tRNA that is complementary to the codon.

• Codon: Example: AUG codes for Methionine (start codon).

• Anti-codon: Example: UAC on tRNA pairs with AUG on mRNA.

Gene Expression

Gene expression determines which proteins are produced in a cell, giving each cell its unique function and structure. All cells contain the same DNA, but express different genes.

• Gene Expression: Regulated to allow cell specialization.

• Homeotic Genes: Master control genes that determine the placement of body parts; mutations can cause abnormal development.

Mutations and Their Effects

A mutation is a change in the nucleotide sequence of DNA. Mutations can affect gene expression and protein production, sometimes resulting in diseases such as cancer.

• Mutation: May cause no protein or abnormal protein to be produced.

• Loss of gene expression control: Can lead to uncontrolled cell growth (cancer).

Mutagens

Mutagens are substances that cause mutations in DNA.

• Examples: Tobacco, radiation, sunlight.

Proto-oncogenes, Oncogenes, and Tumors

Proto-oncogenes code for proteins that regulate cell division. When mutated, they become oncogenes, leading to uncontrolled cell division and tumor formation.

• Proto-oncogenes: Normal genes that promote cell division.

• Oncogenes: Mutated proto-oncogenes causing unchecked cell division.

• Tumors: Can be benign (harmless, isolated) or malignant (spreading, harmful).

Reducing Cancer Risk

Cancer risk can be reduced by avoiding mutagens and promoting healthy cell division.

• Avoid mutagens: Tobacco, radiation, excessive sunlight.

• Promote healthy habits: Eat a balanced diet, exercise regularly, get frequent screenings.

Table: Types of Tumors