Unit_6_Study_Guide__1_

Topic 5: Molecular Genetics

The Molecular Basis of Inheritance

Understanding the structure of DNA is vital as it determines how genetic information is stored and transmitted across generations.

Key studies that provided insights into DNA as genetic material include:

1. Griffith's experiments with Streptococcus pneumoniae:

   • Demonstrated transformation by mixing heat-killed smooth strains with rough strains, suggesting that some heritable material could be transferred between bacteria.

2. Avery, MacLeod, and McCarty (1944):

   • Identified that DNA was the transforming factor in Griffith's experiments, solidifying the notion that DNA is pivotal in heredity.

3. Hershey and Chase (1952):

   • Used bacteriophages and radioactive labeling to show that DNA, rather than protein, served as the genetic material responsible for bacterial infection.

4. Chargaff's Rules:

   • Established that in DNA, adenine pairs with thymine, and cytosine pairs with guanine in equal molar proportions, underscoring the specificity in base pairing crucial for DNA replication and function.

5. Watson and Crick:

   • Developed the double helix model of DNA, explaining its structural features like how the two strands run in opposite directions (antiparallel), reinforcing its role in genetic inheritance.

DNA features:

• Double helix structure: Similar to a twisted ladder, where the rungs are the nitrogenous bases (adenine, thymine, guanine, cytosine).

• Sugar-phosphate backbone: Provides structural integrity and protects the genetic code.

• Antiparallel strands: Strands run in opposite directions (5' to 3' and 3' to 5'), which is essential for the correct replication and transcription processes.

DNA Replication

Semiconservative Replication:

• Each daughter molecule inherits one original strand and one newly synthesized strand, ensuring genetic continuity.

Enzymes involved in replication include:

• DNA polymerase: Responsible for adding nucleotides to the growing strand during replication, ensuring fidelity in base pairing.

• Ligase: Seals gaps between Okazaki fragments on the lagging strand, thus completing the strand.

• Helicase: Unwinds the double helix to allow replication forks to form, preparing the template for new strand synthesis.

• Topoisomerase: Relieves the tension created by the unwinding DNA, preventing supercoiling that could hinder the replication process.

Key Steps of DNA Replication:

1. Initiation: Begins at specific origins of replication where the DNA strands are separated.

2. Formation of a replication bubble: This allows for simultaneous synthesis of both strands.

3. Elongation of new DNA strands: DNA polymerases synthesize new strands by elongating them in the 5' to 3' direction.

4. Continuous leading strand vs. discontinuous lagging strand: The leading strand is synthesized continuously, whereas the lagging strand is synthesized in short fragments (Okazaki fragments).

5. Final sealing: After elongation, DNA ligase seals the gaps ensuring a continuous double-stranded molecule.

Chromosomal Structure and Gene Expression

Chromosomes:

• Bacterial chromosomes: Consist of a single, circular DNA molecule, which is compacted to fit within the cell.

• Eukaryotic chromosomes: Linear and associated with proteins (chromatin), allowing for greater complexity in gene regulation and expression.

Euchromatin vs. Heterochromatin:

• Euchromatin: Extended form of chromatin that is accessible for transcription during interphase.

• Heterochromatin: Condensed form during mitosis, typically inaccessible to transcription machinery, ensuring proper segregation of genetic material during cell division. An example includes Barr bodies, which are inactivated X chromosomes seen in females.

Gene Expression: From Gene to Protein

Central Dogma:

• The flow of genetic information is described by the central dogma: DNA ⇒ RNA ⇒ Protein.

Transcription:

• The process of copying DNA into mRNA occurs in the nucleus, divided into three stages:

  1. Initiation: RNA polymerase binds to the promoter region of a gene.

  2. Elongation: RNA polymerase synthesizes mRNA by adding RNA nucleotides complementary to the DNA template.

  3. Termination: The process ends when RNA polymerase reaches a termination sequence, releasing the newly synthesized mRNA.

• In eukaryotes, pre-mRNA undergoes extensive processing: adding a 5' cap, a poly-A tail, and splicing out introns to produce mature mRNA that will be translated into proteins.

Translation:

• Involves decoding the mRNA into polypeptides at the ribosome located in the cytoplasm. This process relies on codons, triplet codes in mRNA that specify amino acids, enabling the synthesis of proteins. Each tRNA molecule carries a specific amino acid and has an anticodon that pairs with the corresponding mRNA codon, ensuring accurate translation of the genetic code into functional proteins.

Mutation and Gene Regulation

Mutations:

• Point mutations (changes in a single nucleotide):

  • Missense mutations: Result in a change in one amino acid, which may affect protein function.

  • Nonsense mutations: Introduce a premature stop codon, which can truncate the protein's function.

  • Silent mutations: Do not result in a change to the encoded amino acid due to the redundancy of the genetic code.

• Insertions and deletions: These can lead to frameshift mutations that impact the reading frame, often resulting in significant alterations to the encoded protein.

Gene Regulation:

• Operons in prokaryotes: Groups of genes regulated as units (e.g., repressible and inducible operons) which allow for efficient gene expression in response to environmental changes.

• Eukaryotic gene expression: Involves multiple levels of control, including transcription initiation, RNA processing, and post-translation modifications.

• Epigenetic modifications: Such as DNA methylation and histone acetylation influence gene accessibility and expression, providing additional regulatory layers without altering the underlying DNA sequence.

• MicroRNAs (miRNAs): Play a role in regulating gene expression by binding to mRNA, which can block translation or tag mRNA for degradation, adding further control over protein synthesis.