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.

2. 
   

3. Prophase:
   
   

   • Chromosomes condense and become visible.

   • Nuclear envelope breaks down.

   • Spindle fibers begin to form from centrioles.

4. 
   

5. Metaphase:
   
   

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

   • Spindle fibers attach to the centromeres.

6. 
   

7. Anaphase:
   
   

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

8. 
   

9. Telophase:
   
   

   • Nuclear membranes reform around each set of chromosomes.

   • Chromosomes uncoil into chromatin.

10. 
    

11. Cytokinesis:
    
    

    • Cytoplasm divides, forming two identical daughter cells.

12. 
    

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).

2. 
   

3. Metaphase I:
   
   

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

4. 
   

5. Anaphase I:
   
   

   • Homologous chromosomes separate to opposite poles.

6. 
   

7. Telophase I / Cytokinesis:
   
   

   • Two haploid cells form.

8. 
   

Meiosis II

(Like Mitosis)

1. Prophase II:
   
   

   • Spindle forms again in both haploid cells.

2. 
   

3. Metaphase II:
   
   

   • Chromosomes align at the center.

4. 
   

5. Anaphase II:
   
   

   • Sister chromatids separate.

6. 
   

7. Telophase II / Cytokinesis:
   
   

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

8. 
   

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.

2. 
   

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.

4. 
   

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.

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

  • Usually very disruptive to protein structure.

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• 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.