Genetics and Microbial Genetics: DNA Structure, Function, and Gene Transfer

Genetics

A. Structure and Function of DNA

DNA (deoxyribonucleic acid) is the hereditary material in almost all living organisms. Its structure and function are fundamental to understanding genetics and molecular biology.

• Double-stranded (DS), anti-parallel: DNA consists of two strands running in opposite directions, forming a double helix.

• Base pairing rules: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C).

• ATP: Adenosine triphosphate is the energy molecule prominent in cells. In RNA, ATP is the precursor monomer nucleotide, while in DNA, dATP is used.

• Transcription: DNA instructions are transcribed into RNA.

B. Structure and Function of RNA

RNA (ribonucleic acid) is typically single-stranded (SS) and uses Uracil (U) instead of Thymine. The base pairing rules are A-U and G-C. RNA plays several roles in protein synthesis.

• mRNA (messenger RNA): Carries genetic information from DNA to the ribosome, directing protein synthesis.

• tRNA (transfer RNA): Transfers specific amino acids to the ribosome during translation, matching amino acids to the mRNA codons.

• rRNA (ribosomal RNA): Forms the core of the ribosome's structure and catalyzes protein synthesis.

Type	Function
mRNA	Information molecule; directs ribosome to put amino acids in order for a polypeptide chain
tRNA	Transfer molecule; each tRNA carries a specific amino acid to the mRNA/ribosome complex
rRNA	Ribosomal, enzymatic molecule; forms the ribosome with proteins

C. Replication (DNA)

DNA replication is the process by which DNA makes a copy of itself during cell division. It is semi-conservative, meaning each new DNA molecule contains one old and one new strand.

• Step 1: Unwinding - Helicase breaks hydrogen bonds, separating the two DNA strands.

• Step 2: RNA Priming - RNA polymerase (primase) synthesizes a short RNA primer to initiate synthesis.

• Step 3: Elongation - DNA polymerase adds nucleotides to the 3' end, synthesizing a new strand complementary to the template.

• Step 4: Primer Removal and Replacement - DNA polymerase removes RNA primers and replaces them with DNA.

• Step 5: Ligation - DNA ligase joins Okazaki fragments on the lagging strand.

Directionality: DNA synthesis occurs in the 5' to 3' direction.

Fidelity: DNA polymerase has high fidelity, but errors (mutations) can still occur.

D. Transcription

Transcription is the synthesis of RNA from a DNA template. RNA polymerase binds to the start of a gene and synthesizes a single-stranded RNA molecule complementary to the DNA template, in the 5' to 3' direction.

E. Translation

Translation is the process by which ribosomes synthesize proteins using mRNA as a template. It involves decoding the mRNA sequence into a polypeptide chain.

• The ribosome binds mRNA and synthesizes a polypeptide chain according to the mRNA codons.

• Translation starts at the start codon (AUG) and proceeds codon by codon.

• tRNA molecules with complementary anti-codons bring specific amino acids to the ribosome.

• Peptide bonds form between amino acids, and the chain elongates until a stop codon is reached.

Codon Table: Each codon (three nucleotides) codes for a specific amino acid or a stop signal.

F. Gene Regulation (lac operon)

Gene regulation ensures that genes are expressed only when needed. The lac operon is a classic example in bacteria, controlling the metabolism of lactose.

• Constitutive genes: Always on.

• Inducible genes: Turned on in response to environmental signals (e.g., presence of lactose).

• Repression and activation: Genes can be repressed (turned off) or activated (turned on) as needed.

Application: Efficient gene regulation prevents waste and ensures adaptation to environmental changes.

G. Mutation

Mutations are changes in the DNA sequence. They can occur spontaneously or be induced by chemicals or radiation.

• Types of mutations:

  • Spontaneous (errors in DNA replication)

  • Chemical (DNA damage by chemicals)

  • Radiation (DNA damage by radiation)

• Outcomes of mutation:

  • Silent (no change in protein function)

  • Neutral (amino acid change, but no effect on function)

  • Harmful (loss of function or deleterious effect)

  • Beneficial (rare, but can provide an advantage)

Natural selection: Beneficial mutations may be preserved, while harmful ones are less likely to persist.

Microbial Genetics

A. Plasmids

Plasmids are small, circular, autonomously replicating DNA molecules found in bacteria. They often carry genes for antibiotic resistance or other traits and can be transferred between cells.

B. Mechanisms for Taking on New Genetic Information

Bacteria can acquire new genetic information through several mechanisms, distinct from mutation:

1. Transformation

Uptake of free DNA from the environment by a bacterial cell. Discovered by Griffith using Streptococcus pneumoniae. Transformation is relatively rare and often requires specific conditions.

• Competent cells can take up DNA and incorporate it into their genome.

• Used in biotechnology for genetic engineering.

2. Conjugation

Transfer of DNA from one cell to another via direct contact, usually mediated by a pilus. The F (fertility) plasmid system in Escherichia coli is a well-studied example.

• F+ cells: Contain the F plasmid and can transfer it to F- cells.

• Hfr cells: High frequency of recombination; F plasmid integrated into the chromosome, can transfer chromosomal genes.

Type	Characteristics
F+ cells	Have a plus (F) plasmid; can transfer DNA to F- cells
Hfr cells	F plasmid integrated into chromosome; can transfer chromosomal genes

Resistance factors: Plasmids may carry genes for antibiotic resistance, toxin production, or other survival advantages.

3. Transduction

Transfer of bacterial DNA from one cell to another via bacteriophages (viruses that infect bacteria).

• Generalized transduction: Any bacterial gene can be transferred; occurs when a phage accidentally packages host DNA.

• Specialized transduction: Only specific genes near the prophage insertion site are transferred; occurs with temperate phages during lysogenic cycle.

Type	Description
Generalized	Any gene from donor can be transferred; not limited to specific loci
Specialized	Only genes adjacent to prophage site are transferred

Similarities: Both describe viral transfer of cellular DNA from one cell to another.

Differences: Generalized can transfer any gene; specialized is limited to certain genes.

Summary Table: Mechanisms of Horizontal Gene Transfer in Bacteria

Mechanism	Key Features	Example
Transformation	Uptake of free DNA from environment	Streptococcus pneumoniae
Conjugation	Direct cell-to-cell transfer via pilus	F plasmid in E. coli
Transduction	DNA transfer via bacteriophage	Generalized and specialized transduction

Applications and Importance

• Horizontal gene transfer contributes to genetic diversity and evolution in bacteria.

• Plasmids and transduction are important in the spread of antibiotic resistance.

• Genetic engineering utilizes these mechanisms for biotechnology and research.

Additional info: The notes reference figures and chapters from a textbook for further reading and context, such as codon tables and diagrams of gene transfer mechanisms.