chapter 7 pt 2

Chapter 7: Blueprint of Life - DNA and Proteins

DNA Replication

• DNA Ligase

  • An enzyme that repairs irregularities in the backbone of double-stranded DNA.

  • Functions:

    • Seals repairs in DNA.

    • Seals recombination fragments.

    • Connects Okazaki fragments (small DNA fragments formed during replication).

  • Mechanism: Forms a bond between the end of a donor nucleotide and an acceptor nucleotide.

Okazaki Fragments

• Synthesis Direction:

  • DNA is synthesized in the 5' to 3' direction but read in the 3' to 5' direction.

  • DNA polymerase synthesizes the leading strand continuously and the lagging strand in fragments.

• Discovery: Named after researcher Okazaki, as fragments are generated when synthesizing the lagging strand.

• RNA Primers:

  • Required to initiate the formation of Okazaki fragments.

  • Removed by DNA polymerase after fragment synthesis.

  • Joining of Okazaki fragments is completed with DNA ligase.

Genetic Code and Translation

• Codons:

  • A sequence of three nucleotides in DNA and RNA corresponding to a specific amino acid.

  • Represents the relation between DNA bases (A, C, G, T) and the corresponding protein sequence.

• Anticodons:

  • Sections of tRNA with three bases that correspond to codons in mRNA.

  • Establish base pairs with codon counterparts during translation.

• Function:

  • Anticodons ensure the correct amino acids are attached to proteins during synthesis based on mRNA instructions.

Comparison: Codons vs Anticodons

• Codon:

  • Composed of three nucleotides in mRNA.

  • Codes for a specific amino acid.

• Anticodon:

  • Composed of three nucleotides in tRNA that pair with corresponding codons in mRNA.

The Lac Operon

• Definition:

  • A sequence of DNA that contains genes coding for enzymes in protein synthesis.

• Components:

  • Regulatory Elements: Control gene expression.

  • Structural Elements: Include genes such as lacZ, lacY, lacA.

Structure of the Lac Operon

• Promoter Region:

  • Controls transcription of structural genes, regulated by repressor genes.

• Operator Region:

  • Functions as a switch for transcription.

  • Regulatory gene binds here to influence transcription.

• Regulator Region:

  • Produces repressor proteins which interact with both promoter and operator.

Gene Expression in Prokaryotes vs Eukaryotes

• Prokaryotes:

  • Transcription and translation occur simultaneously in the cytoplasm, enabling rapid responses.

• Eukaryotes:

  • Transcription occurs in the nucleus, translation in the cytoplasm, necessitating RNA processing.

Control of Gene Expression

• Prokaryotic Control Mechanisms:

  • Positive Control:

    • Inducible system, regulates gene expression in response to lactose presence.

  • Negative Control:

    • Involves the repression of gene expression in absence of lactose.

Positive Control of Lac Operon

• Mechanism:

  • Regulatory gene is expressed, producing repressor proteins.

  • Lactose binds to repressor, preventing it from blocking transcription, allowing RNA polymerase to transcribe mRNA.

Negative Control of Lac Operon

• Mechanism:

  • In absence of lactose, the repressor binds to the operator, blocking RNA polymerase, inhibiting mRNA transcription.

Inducer (Antirepressor)

• Function:

  • Suppresses repressor activity, making it inactive and allowing transcription to proceed.

  • Allows the operon to be turned on and off.

Resources

• Video Resource:

  • DNA Replication: Enzymes involved (DNA Polymerase, Helicase, Primase, Topoisomerase)