Biology Notes: DNA, Transcription, Translation, and Mutations

Definition of DNA
- DNA stands for Deoxyribonucleic Acid.
- It is composed of nucleotides which include nitrogenous bases and the backbone structure.
Components of Nucleotides
- A nucleotide consists of three parts:
- Deoxyribose Sugar
  - A five-carbon sugar that forms the backbone of DNA.
- Nitrogenous Base
  - Contains nitrogen and is classified as basic.
  - There are four types of nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T).
- Phosphate Group
  - Linked to the deoxyribose sugar, contributing to the sugar-phosphate backbone of DNA.
Types of Nitrogenous Bases
- Bases are classified into two categories:
- Pyrimidines
  - Cytosine (C) and Thymine (T) are pyrimidines with a single ring structure.
- Purines
  - Adenine (A) and Guanine (G) have a double ring structure.
DNA Structure
- Double Helix
- DNA consists of two strands that form a helix.
- The strands are held together by hydrogen bonds between complementary bases:
  - A pairs with T.
  - C pairs with G.
- Antiparallel Orientation
- One strand runs 5' to 3', while the other runs 3' to 5'.
- This orientation is crucial for DNA replication.

Semiconservative Replication
- DNA replication is termed semiconservative: each resulting DNA molecule has one original strand and one newly synthesized strand.
Stages of DNA Replication
1. Initiation
- Occurs at a specific region called the Origin of Replication (OriC).
- Enzymes and proteins assemble to form replication factories, primarily DNA polymerase, which synthesizes new DNA strands.
1. Elongation
- DNA polymerase adds nucleotides complementary to the template strand, forming a new strand in the replication bubble.
- Synthesis occurs in both directions until replication bubbles meet.
1. Termination
- Completion of DNA replication results in two identical double-stranded DNA molecules, each with one original and one new strand.
Replication Fork
- The replication fork is the site where DNA strands are separated and actively replicated.
- Leading Strand vs Lagging Strand:
- Leading Strand
  - Synthesized continuously in the 5' to 3' direction.
- Requires only one RNA primer.
- Lagging Strand
  - Synthesized discontinuously in short segments called Okazaki fragments due to the antiparallel structure.
  - Requires multiple RNA primers for each fragment.
  - DNA Ligase joins the Okazaki fragments together post-replication.
Key Enzymes
- Helicase: unwinds the DNA double helix.
- DNA Ligase: seals gaps between Okazaki fragments.
- Single-Stranded Binding Proteins: stabilize unwound DNA.

Definition of Transcription
- Transcription is the process of synthesizing RNA from a DNA template.
RNA Structure
- Unlike DNA, RNA is single-stranded and contains ribose sugar instead of deoxyribose.
- The nitrogenous base uracil (U) replaces thymine (T) found in DNA.
Types of RNA
- Messenger RNA (mRNA)
- Carries the genetic message from DNA to ribosomes for protein synthesis.
- Transfer RNA (tRNA)
- Brings amino acids to the ribosome during translation.
- Ribosomal RNA (rRNA)
- Forms the core structural and functional components of ribosomes.
RNA Polymerase
- Enzyme responsible for synthesizing RNA.
- Binds to the promoter region of DNA to initiate transcription.
Steps of Transcription
1. Unwinding
- The DNA helix unwinds to expose the template strand.
1. Binding
- RNA polymerase attaches to the promoter, determining the direction and strand to be transcribed.
1. Polymerization
- RNA polymerase assembles RNA nucleotides in the 5' to 3' direction until it reaches a terminator sequence, signaling the end of transcription.
Processing of Eukaryotic mRNA
- Eukaryotic mRNA undergoes post-transcriptional modifications such as adding a 5' cap and poly-A tail, and splicing to remove introns.
- Monocistronic vs Polycistronic: Eukaryotic mRNA typically encodes one protein (monocistronic) while prokaryotic mRNA can encode multiple proteins (polycistronic).

Definition of Translation
- Translation is the process of synthesizing a polypeptide chain (protein) based on the sequence of nucleotides in mRNA.
Genetic Code
- The genetic code consists of triplet codons that correspond to specific amino acids.
- This triplet nature allows for 64 possible combinations (4^3), sufficient to code for 20 amino acids.
Properties of the Genetic Code
- Degeneracy: Multiple codons can encode the same amino acid.
- Unambiguity: Each codon corresponds to only one amino acid.
- Start and Stop Signals: AUG is the start codon; UAA, UAG, and UGA are stop codons.
- Universality: The genetic code is nearly universal across all living organisms.
Ribosome Structure
- Composed of large and small subunits with binding sites for mRNA and tRNA.
- The three primary binding sites are the A (aminoacyl), P (peptidyl), and E (exit) sites.
Steps of Translation
1. Initiation: The small ribosomal subunit binds to mRNA and the initiator tRNA binds to the start codon (AUG).
2. Elongation: Successive tRNA molecules bring amino acids to the ribosome, forming peptide bonds and lengthening the polypeptide chain.
3. Termination: The ribosome encounters a stop codon, ceasing translation and releasing the completed polypeptide.

Definition of Mutations
- Mutations are changes in the nucleotide sequence of DNA that can affect protein synthesis and function.
Types of Mutations
- Point Mutations:
- Involves a change in a single base pair. Can result in:
  - Silent Mutation: No change in amino acid sequence.
  - Missense Mutation: Changes one amino acid, potentially affecting protein function (e.g., sickle cell anemia).
  - Nonsense Mutation: Converts a codon to a stop codon, leading to a truncated protein.
- Insertions and Deletions:
- Involve the addition or removal of nucleotides, often resulting in frameshift mutations that alter the reading frame and significantly change downstream amino acid sequences.
Consequences of Mutations
- Mutations can be neutral, beneficial, or harmful, depending on their impact on protein function and expression.