Genetics: Structure, Function, and Expression of Genetic Material in Microbiology

Genotype and Phenotype

Definitions and Historical Context

Genetics is the study of genes, their function, and how variations arise in genomes. The genome is the entire collection of genetic material in a cell or virus. By the early 1900s, the term gene was used to describe heritable units of genetic material that determine particular traits. The genotype (genetic makeup) influences the phenotype (physiological and physical traits) of an organism. Genomes can be thought of as instruction manuals that determine all possible features of a cell or virus.

• Cells have deoxyribonucleic acid (DNA) genomes.

• Viruses may have either DNA or ribonucleic acid (RNA) genomes.

Prokaryotic and Eukaryotic Genomes

Size and Organization

Generally, more complex organisms have more genes. Genomes are organized into chromosomes, which are strands of DNA associated with organizational proteins. The number of chromosomes does not determine organism sophistication.

• Eukaryotic cells: Numerous linear chromosomes in the nucleus; DNA is organized by histones.

• Prokaryotic cells: Usually a single circular chromosome in the nucleoid region; DNA is organized by histone-like proteins.

Some cells also contain plasmids—pieces of DNA outside the chromosomal DNA that often confer survival advantages (e.g., antibiotic resistance).

Table: Genome Characteristics

Factor	Prokaryotic Genomes	Eukaryotic Genomes
Complexity	Simple	More complex
Genome can include	Chromosomal DNA and plasmids	Chromosomal DNA, plasmids, and DNA in mitochondria and chloroplasts
Chromosomes	Few (generally only one); usually circular	Many; nuclear chromosomes are linear
Location of chromosome(s)	Nucleoid region	Nucleus
DNA organized by	Histone-like proteins	Histones

The Nucleic Acids DNA and RNA

Structure and Function

Nucleic acids (DNA and RNA) govern all aspects of cell life, from structure to function. They are built from nucleotides, each consisting of:

• Phosphate group

• Sugar (deoxyribose in DNA, ribose in RNA)

• Nitrogen base (Adenine, Guanine, Cytosine, Thymine in DNA; Uracil replaces Thymine in RNA)

Nitrogen bases are classified as purines (A, G) or pyrimidines (C, T, U). Base pairing rules: A-T (DNA), A-U (RNA), G-C (both).

Table: DNA and RNA Nitrogen Bases

Nitrogen Base	Family	Pairs with	Found in
Adenine (A)	Purine	Thymine (T) or Uracil (U)	DNA and RNA
Guanine (G)	Purine	Cytosine (C)	DNA and RNA
Cytosine (C)	Pyrimidine	Guanine (G)	DNA and RNA
Thymine (T)	Pyrimidine	Adenine (A)	Only DNA
Uracil (U)	Pyrimidine	Adenine (A)	Only RNA

DNA is a double-stranded helix with complementary base pairs forming the "stairs" and a sugar-phosphate backbone forming the "railings". The strands are held together by phosphodiester bonds and have antiparallel orientation (one runs 5' to 3', the other 3' to 5').

RNA Structure

• RNA nucleotides are called ribonucleotides.

• RNA is usually single-stranded, contains ribose sugar, and uses uracil instead of thymine.

• Types of RNA: Messenger RNA (mRNA), Transfer RNA (tRNA), Ribosomal RNA (rRNA).

Central Dogma: Flow of Genetic Information

The central dogma of molecular biology states that genetic information flows from DNA to RNA to protein. DNA is transcribed into RNA, which is then translated into protein.

DNA Replication

Process and Enzymes

DNA replication is the process by which a cell copies its genome before division. It involves unwinding the original DNA, copying it, and rewinding the parent and new DNA. Replication is highly accurate due to base-pairing rules and proofreading by DNA polymerases.

Table: Key Enzymes in DNA Replication

Enzyme	Function
Helicase	Unwinds DNA helix
Primase	Builds RNA primers
DNA polymerase III	Main enzyme that copies DNA
DNA polymerase I	Replaces RNA primers with DNA
Ligase	Forms phosphodiester bonds to seal nicks
Gyrase and Topoisomerases	Relieve tension from unwinding

Leading vs. Lagging Strand

• Leading strand: Synthesized continuously in the same direction as the replication fork.

• Lagging strand: Synthesized discontinuously in segments called Okazaki fragments, opposite to the fork direction.

DNA replication is semiconservative: each new DNA molecule contains one original and one new strand.

Protein Synthesis: Transcription and Translation

Transcription

Transcription is the process of making RNA from DNA. It occurs in the nucleus (eukaryotes) or cytoplasm (prokaryotes) and involves three steps:

1. Initiation: RNA polymerase binds to the promoter and unwinds DNA.

2. Elongation: RNA polymerase adds complementary ribonucleotides (A-U, G-C).

3. Termination: RNA polymerase reaches a termination sequence and releases the RNA transcript.

Translation

Translation is the process by which ribosomes decode mRNA to build proteins. The genetic code is read in triplets (codons), each coding for an amino acid or stop signal. There are 64 codons, encoding 20 standard amino acids and stop signals. The process involves:

1. Initiation

2. Elongation

3. Termination

Protein synthesis is essential for cell function and is a target for many antibiotics.

Additional info:

• Splicing of mRNA (in eukaryotes) removes introns and joins exons, allowing for alternative splicing and protein diversity.

• Reverse transcription (in some viruses) uses RNA as a template to make DNA.