DNA Replication Processes

DNA Replication

• DNA replication involves duplicating a double-stranded DNA molecule, creating a copy of itself.
• This occurs during the interphase stage of cell division, specifically mitosis.
• Replication is crucial to ensure each daughter cell receives a copy of the parent DNA molecule's genetic information.
• The genetic information on the DNA molecule must be retained identically during replication.

DNA Replication Processes

• DNA unwinds and separates into two strands.
• Each strand acts as a template for synthesizing new complementary strands.
• Bases are added one at a time until two new DNA strands are created, precisely duplicating the original DNA.

Semi-Conservative Replication

• DNA replication is a semi-conservative process.
• The parent DNA strand separates into two strands.
• Each strand serves as a template for a new complementary strand.
• The newly formed double helix is half original DNA from the parent and half new DNA
• The semi-conservative model was established after Meselson and Stahl's 1958 experiments.

Mechanism of DNA Replication

• Helicases: Unwind the parent double helix.
• Single-strand binding proteins: Stabilize the unwound DNA.
• DNA polymerase: Synthesizes new DNA strands in a 5' to 3' direction.
• Leading strand: Synthesized continuously.
• Lagging strand: Synthesized discontinuously in short fragments (Okazaki fragments).
• DNA polymerase I: Replaces RNA primers with DNA.
• DNA ligase: Joins Okazaki fragments.
• Topoisomerase: Unknots and uncoils DNA to relieve unwinding strain.
• Primase: Creates RNA primers for DNA polymerase to start replication.

Meselson-Stahl Experiment

• Provided evidence that disproves the conservative and dispersive models of DNA replication.

Conservative Hypothesis

• Proposed that both parent DNA strands remain intact as a template to create a new DNA strand.

Dispersive Hypothesis

• Proposed that the new DNA molecule is made of fragments of older and new DNA strands.

Protein Synthesis and Stages

• Genetic information in DNA is used to synthesize polypeptides (proteins).
• Protein synthesis happens in two stages: Transcription and Translation.

Transcription

• Reads the DNA genetic code and transfers it to messenger RNA (mRNA).

Translation

• Converts the genetic code into a protein.

Genetic Code

• A sequence of bases on a gene.
• A gene is a sequence of bases that codes for a specific amino acid.
• A combination of three different bases (nucleotides) in a triplet sequence that codes for the 20 amino acids.
• A triplet of bases on the mRNA (a codon) specifies a particular amino acid.
• Codons are written 5' to 3'.
• AUG is an initiation codon and codes for methionine.
• UAA, UAG, and UGA are stop codons.
• The genetic code consists of 64 (4³) triplet base sequences or codons.

Degeneracy of the Genetic Code

• Some amino acids are specified by more than one codon.
• The genetic code is degenerate, meaning it has more codes than needed.

Transcription

• Transcription takes place similarly to DNA replication, with the double-stranded structure unwinding and one strand serving as a template for RNA formation.
• Unwinding is done by DNA helicase, breaking hydrogen bonds.
• The DNA strand used as a template is called the anti-sense strand.
• The corresponding mRNA created is called the sense strand.

Transcription Start Process

• Transcription is triggered when RNA polymerase joins to a sigma protein.
• RNA polymerase then binds to a promoter sequence in a DNA molecule.
• RNA polymerase reads the DNA anti-sense strand in the 3' to 5' direction, creating an mRNA sequence in the 5' to 3' direction

mRNA and Translation

• Three bases on the DNA strand match corresponding three bases on the mRNA (codon), e.g. ATC on DNA corresponds to UAG on mRNA
• mRNA moves to the cytoplasm to find a ribosome, and the DNA rewinds.
• mRNA moves out of the nucleus into the cytoplasm to ribosomes for translation to happen.

Translation

• mRNA travels to ribosomes in the cytoplasm.
• The genetic material (codons) on mRNA is interpreted to create amino acids(protein).
• Ribosomal RNA dissociates into small and large subunits.
• Large Ribosomal units consist of amino acyl site (A) and peptidyl site (P),where amino acids are joined by peptide bonds.
• The anticodon of transfer RNA (tRNA) reads the codon on the mRNA. Each codon represents an amino acid.
• Amino acids are attached at the 3' end of tRNA with aid from ATP and specific enzyme complexes(aminoacyl-tRNA synthetases).
• Ribosomes read mRNA in the same 5' to 3' direction as it was polymerized.
• Each of the 20 amino acids has their own type of tRNA to operate in a cell.

This quiz explores the mechanisms of DNA replication, including the semi-conservative model and the stages involved. Understand how DNA unwinds, separates, and synthesizes new strands during cell division. Test your knowledge on this fundamental biological process.