Semester 2 Final

1. DNA (Deoxyribonucleic Acid)

  • What it is: A molecule that contains the genetic blueprint for life.

  • Structure: Double helix, composed of nucleotides (A, T, C, G).

  • Function: Stores genetic information used to build and maintain an organism.

2. RNA (Ribonucleic Acid)

  • What it is: A single-stranded molecule that helps carry out the instructions in DNA.

  • Types:

    • mRNA (messenger RNA): Carries DNA’s message to ribosomes.

    • tRNA (transfer RNA): Brings amino acids to the ribosome.

    • rRNA (ribosomal RNA): Forms part of the ribosome’s structure.

  • Function: Essential for protein synthesis.

3. Mitosis – Detailed

Purpose: To produce two genetically identical diploid cells (2n), used for growth, repair, and asexual reproduction.

Steps:

  1. Interphase (Pre-Mitosis):

    • DNA replicates in the S phase.

    • Chromosomes are not yet visible; they are in chromatin form.

  3. Prophase:

    • Chromosomes condense and become visible.

    • Nuclear envelope breaks down.

    • Spindle fibers begin to form from centrioles.

  5. Metaphase:

    • Chromosomes align at the metaphase plate (center of the cell).

    • Spindle fibers attach to the centromeres.

  7. Anaphase:

    • Sister chromatids are pulled apart to opposite poles of the cell.

  9. Telophase:

    • Nuclear membranes reform around each set of chromosomes.

    • Chromosomes uncoil into chromatin.

  11. Cytokinesis:

    • Cytoplasm divides, forming two identical daughter cells.

4. Meiosis – Detailed

Purpose: To produce haploid gametes (n), with half the chromosome number of the parent cell. Increases genetic diversity.

Key Features:

  • Two divisions: Meiosis I and Meiosis II.

  • Involves crossing over and independent assortment.

Meiosis I

(Reduction Division)

  1. Prophase I:

    • Homologous chromosomes pair up (synapsis) and exchange DNA segments (crossing over).

  3. Metaphase I:

    • Homologous pairs align in the middle; independent assortment occurs.

  5. Anaphase I:

    • Homologous chromosomes separate to opposite poles.

  7. Telophase I / Cytokinesis:

    • Two haploid cells form.

Meiosis II

(Like Mitosis)

  1. Prophase II:

    • Spindle forms again in both haploid cells.

  3. Metaphase II:

    • Chromosomes align at the center.

  5. Anaphase II:

    • Sister chromatids separate.

  7. Telophase II / Cytokinesis:

    • Four non-identical haploid cells (gametes) are formed.

5. Gene Expression – Detailed

Definition: The process by which the instructions in DNA are used to synthesize a functional product (usually a protein).

Two Major Steps:

  1. Transcription (DNA → mRNA):

    • Occurs in the nucleus.

    • Enzyme RNA polymerase reads a gene’s DNA sequence and builds a complementary mRNA strand.

    • mRNA then exits the nucleus.

  3. Translation (mRNA → Protein):

    • Occurs at a ribosome in the cytoplasm or on the rough ER.

    • mRNA codons are read 3 bases at a time.

    • Each codon codes for one amino acid.

    • tRNA brings the correct amino acid, matching its anticodon to the mRNA codon.

Regulation:

  • Controlled by transcription factors, repressors, enhancers, and epigenetic modifications (e.g., methylation).

6. Mutations – Detailed

Definition: A change in the nucleotide sequence of DNA.

Types:

  • Point Mutation: One base pair is changed (e.g., A → G).

    • Silent: No change in amino acid.

    • Missense: Different amino acid.

    • Nonsense: Early stop codon.

  • Frameshift Mutation: Insertion or deletion of a base shifts the reading frame.

    • Usually very disruptive to protein structure.

  • Chromosomal Mutations: Large-scale changes like deletions, duplications, inversions, or translocations.

Causes:

  • Errors in replication.

  • Radiation, chemicals, viruses.

Effects:

  • Neutral, harmful (e.g., cancer, genetic disorders), or beneficial (basis of evolution).

7. Protein Synthesis – Detailed

Overview: The biological process of building proteins based on DNA instructions.

Step 1: Transcription (in the nucleus):

  • DNA → mRNA

  • RNA polymerase binds to the promoter region.

  • Builds an mRNA strand complementary to the gene.

  • mRNA undergoes splicing (removal of introns, keeping exons) and receives a 5’ cap and poly-A tail.

Step 2: Translation (in the cytoplasm, at the ribosome):

  • mRNA binds to the ribosome.

  • tRNA molecules bring specific amino acids, each matching a codon on the mRNA with its anticodon.

  • Amino acids are linked by peptide bonds to form a polypeptide chain (protein).

  • Process continues until a stop codon is reached.