DNA Replication and Transcription Notes

Overview of DNA Replication and Transcription

General Concepts

• DNA Replication: The process of copying DNA to produce two identical molecules.
• Transcription: The synthesis of RNA from a DNA template.
• Enzyme Roles: Different enzymes (DNA polymerase for replication, RNA polymerase for transcription) are involved, but the underlying principles are similar.

DNA Replication Details

• Mechanism of Replication:

• DNA strands are anti-parallel; one strand runs from 5' to 3', while the complementary strand runs from 3' to 5'.

• DNA polymerase is the primary enzyme that synthesizes new DNA strands.

• Template Use: One strand serves as a template for replication. The enzyme brings complementary nucleotides in response to the template (A pairs with T, G pairs with C).

• Key Steps in DNA Replication:

1. Separation of Strands: The double helix is unwound.
2. Binding of DNA Polymerase: DNA polymerase can only attach to a 3' OH tail on a template strand.
3. Synthesis of New Strands: Nucleotides are added in the 5’ to 3’ direction, allowing the complementary strand to be formed.

• Leading and Lagging Strands:
• There are leading and lagging strands during replication. The leading strand is continuously synthesized, while the lagging strand is synthesized in fragments (Okazaki fragments).

Transcription Process

• Definition and Purpose: Transcription is the process of creating RNA from a DNA template. The main product is messenger RNA (mRNA).

• Enzyme Involvement: RNA polymerase binds to the promoter region of the DNA and synthesizes RNA.

• Single-Strand Transcription: Only one strand of DNA is transcribed into RNA, depending on where the promoter is located.

• Steps in Transcription:

1. Locating the Promoter: The RNA polymerase binds to the promoter of the coding strand (5' to 3' direction).
2. Synthesis of mRNA: Unlike DNA, RNA contains uracil (U) instead of thymine (T).

Gene Definition

• Definition of a Gene: A gene is a sequence of DNA that is transcribed into RNA, which can either be translated into a protein (mRNA) or act as non-coding RNA (ncRNA) involved in regulation.

Translation Overview

• Process Description: The conversion of mRNA into a polypeptide chain (protein).
• Codons: The mRNA is read in triplets, called codons, where each codon specifies an amino acid.
• Genetic Code Characteristics:
• Specificity: Each triplet (codon) corresponds to a specific amino acid.
• Degeneracy: Multiple codons can code for the same amino acid (e.g., UUU and UUC both code for Phenylalanine).
• Start Codon: Typically AUG, which codes for Methionine and signifies the start of translation.
• Stop Codons: UAA, UAG, UGA indicate the termination of translation.

Summary of Key Differences in Processes

• DNA Replication: Results in two identical DNA molecules; both strands of DNA are copied.
• Transcription: Produces RNA from DNA; one strand is used as a template and the end result is mRNA that reflects the coding sequence (with U instead of T).
• Translation: Converts mRNA into protein; codons in mRNA are read to assemble amino acids into a polypeptide chain.

Closing Remarks

• Understanding these molecular processes is crucial for grasping how genetic information is stored, expressed, and regulated in living organisms. This includes knowing the roles of various enzymes and the structural features of nucleic acids that facilitate replication and transcription.