Genetic Material and DNA Structure

DNA as Genetic Material

DNA is the primary genetic material in most organisms, with the exception of some viruses that use RNA. The genetic material carries the information necessary for the structure, function, and regulation of an organism's cells and is passed from one generation to the next.

• Basic Building Blocks: DNA is composed of nucleotides.

Structure of DNA

The structure of DNA is a double helix formed by two strands of nucleotides. Each strand has a sugar-phosphate backbone and nitrogenous bases.

• Double Helix: Two strands of nucleotides form a double helix.

• Sugar-Phosphate Backbone: Provides structural support.

• Antiparallel Strands: The two strands run in opposite directions (5' to 3' and 3' to 5').

• Base Pairing: Complementary base pairing (A-T, G-C) is held together by hydrogen bonds.

DNA Replication

Overview of DNA Replication

DNA replication is the process by which a cell makes an exact copy of its DNA. In bacteria, DNA is often closed and circular.

• Genomic DNA: Refers to the complete genetic material in an organism.

• Semiconservative Replication: Each new DNA molecule consists of one old (parental) strand and one newly synthesized strand.

Major Requirements for Replication

• Template: A single-stranded DNA template is required.

• RNA Primer: Short RNA sequences that provide a starting point for DNA synthesis.

• Raw Materials: Deoxynucleoside triphosphates (dNTPs) are assembled into the new DNA strand.

• Enzymes: DNA polymerase is the main enzyme that synthesizes new DNA by reading the template and assembling nucleotides.

Direction of Replication

• New nucleotides are joined one at a time to the 3' end of the growing strand.

• DNA polymerases can only add nucleotides to the 3' end.

• DNA strand elongation always proceeds 5' to 3'.

• Leading Strand: Synthesized continuously.

• Lagging Strand: Synthesized discontinuously in short fragments (Okazaki fragments).

Additional info: The antiparallel nature of DNA strands means that synthesis on one template proceeds left to right, while on the other it must proceed right to left.

Genetic Variation and Gene Transfer

Genetic Variation

Genetic variation results from changes in the genetic material and is essential for evolution and adaptation.

Vertical vs. Horizontal Gene Transfer

• Vertical Gene Transfer: Transmission of genetic material from parent to offspring.

• Horizontal Gene Transfer: Transfer of genetic material between organisms that are not parent and offspring.

Mechanisms of Genetic Transfer in Bacteria

• Transformation: Uptake of free DNA from the environment.

• Conjugation: Direct transfer of DNA between bacteria via cell-to-cell contact.

• Transduction: Transfer of DNA from one bacterium to another via bacteriophages (viruses that infect bacteria).

Flow of Genetic Information (Protein Synthesis)

Central Dogma of Molecular Genetics

The central dogma describes the flow of genetic information: DNA is transcribed into RNA, which is then translated into protein.

• DNA → mRNA → protein

• Transcription: DNA to RNA

• Translation: RNA to protein

Transcription

Transcription is the process by which a DNA template is used to synthesize a complementary strand of RNA.

• Differences between RNA and DNA: RNA contains ribose sugar and uracil instead of thymine.

• Three Types of RNA:

  • mRNA (messenger RNA): Carries genetic information from DNA to ribosomes for protein synthesis.

  • rRNA (ribosomal RNA): Structural and functional component of ribosomes.

  • tRNA (transfer RNA): Transfers amino acids to ribosomes during protein synthesis; contains the anticodon complementary to mRNA codons.

Steps in Transcription

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

2. Elongation: RNA is synthesized in the 5' to 3' direction.

3. Termination: Transcription ends when a terminator sequence is reached.

Genetic Code and Translation

Genetic Code

The genetic code is a set of rules by which the sequence of nucleotides in mRNA is translated into the sequence of amino acids in a protein.

• Codons: Triplets of nucleotides on mRNA that specify amino acids.

• There are 64 codons: 61 code for amino acids, 3 are stop codons.

• The code is nearly universal among organisms.

Translation

Translation is the process by which the information in mRNA is decoded to build a functional protein. It occurs in the ribosome, which consists of a small and a large subunit.

• tRNA: Brings amino acids to the ribosome; each tRNA has an anticodon that pairs with the mRNA codon.

• rRNA: Forms the core of the ribosome's structure and catalyzes peptide bond formation.

Stages of Translation

1. Initiation:

   • Small ribosomal subunit binds to mRNA, followed by the large subunit.

   • Initiator tRNA binds to the start codon (AUG).

2. Elongation:

   • mRNA is read 5' to 3'.

   • tRNAs bring amino acids to the ribosome, matching codons with anticodons.

   • Peptide bonds are formed between amino acids by peptidyl transferase.

3. Termination:

   • Stop codons signal the end of translation.

   • Polypeptide chain is released.

Gene Regulation in Prokaryotes

Operon Model

Prokaryotic gene regulation often involves operons, which are clusters of genes under the control of a single promoter and operator.

• Structural Genes: Code for proteins.

• Control Region: Includes the promoter (where RNA polymerase binds) and operator (where regulatory proteins bind).

• Regulatory Gene: Produces a repressor protein that can bind to the operator to inhibit transcription.

Inducible Operons (e.g., Lac Operon)

• Lac Operon: Controls the metabolism of lactose in bacteria.

• When lactose is absent, the repressor binds to the operator, blocking transcription.

• When lactose is present, it binds to the repressor, inactivating it and allowing transcription.

• Cells prefer glucose over lactose; when glucose is present, the lac operon is not transcribed even if lactose is available.

Mutations and Genetic Errors

Mutations

Mutations are changes in the genetic material of a cell. They can be neutral, beneficial, or harmful.

• Mutagens: Agents that cause mutations (e.g., chemicals, radiation).

• Spontaneous Mutations: Occur without the presence of a mutagen.

Types of Mutations

• Point Mutations: Changes in a single nucleotide.

  • Substitutions: Replacement of one nucleotide with another.

  • Missense Mutations: Change results in a different amino acid.

  • Nonsense Mutations: Change results in a stop codon.

• Insertions and Deletions:

  • Insertion: Addition of one or more nucleotides.

  • Deletion: Loss of one or more nucleotides.

  • Can cause frameshift mutations, altering the reading frame and usually resulting in nonfunctional proteins.

  • Exception: Insertions or deletions of a group of three nucleotides may not cause a frameshift.

Example: Sickle cell anemia is caused by a point mutation in the gene encoding the beta chain of hemoglobin.