Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Explain DNA replication and its significance.
• Describe the process of transcription (DNA to RNA).
• Describe the process of translation (RNA to protein).
• Understand the genetic code, including start and stop codons.
• Explain the central dogma: DNA → RNA → Protein.

Lesson summary

This week delves deeper into DNA, the molecule holding the blueprint of life. Understanding DNA is crucial not just for biology class, but also for understanding diseases like HIV/AIDS prevalent in South Africa, genetic predispositions to conditions like diabetes common in our communities, and even tracing ancestry, which is increasingly popular among South Africans seeking to reconnect with their heritage. DNA technology impacts agriculture, medicine, and even forensic science, areas where South Africa is actively developing its expertise. Without a solid foundation in DNA principles, grasping more advanced topics like genetic engineering and evolution becomes significantly harder.

Lesson notes

A. DNA Replication: DNA replication is the process by which a DNA molecule is copied to produce two identical DNA molecules. This is essential for cell division (mitosis and meiosis), ensuring that each daughter cell receives a complete and accurate copy of the genetic information. The process is semi-conservative, meaning each new DNA molecule consists of one original (template) strand and one newly synthesized strand.

Initiation: Replication begins at specific sites called origins of replication. In eukaryotes (like humans), there are multiple origins of replication along each chromosome, allowing for faster replication.

Unwinding: The enzyme helicase unwinds the double helix by breaking the hydrogen bonds between the complementary base pairs (A-T, C-G). This creates a replication fork, a Y-shaped structure where DNA synthesis occurs. Single-stranded binding proteins (SSBPs) bind to the separated strands to prevent them from re-annealing.

Primer Synthesis: DNA polymerase, the enzyme responsible for adding nucleotides, can only add nucleotides to an existing 3'-OH group.

Therefore, an RNA primer, a short sequence of RNA nucleotides, is synthesized by the enzyme primase. The primer provides the necessary starting point for DNA polymerase.

Elongation: DNA polymerase adds free nucleotides complementary to the template strand, following the base-pairing rules (A with T, C with G). DNA polymerase moves along the template strand in the 3' to 5' direction, synthesizing the new strand in the 5' to 3' direction.

Leading Strand: The leading strand is synthesized continuously in the 5' to 3' direction towards the replication fork. Only one primer is required for its synthesis.

Lagging Strand: The lagging strand is synthesized discontinuously in short fragments called Okazaki fragments, away from the replication fork. Each Okazaki fragment requires a new RNA primer.

Primer Removal: Once the Okazaki fragments are synthesized, the RNA primers are removed by another DNA polymerase (or an enzyme with similar function) and replaced with DNA nucleotides.

Joining: The enzyme DNA ligase joins the Okazaki fragments together by forming phosphodiester bonds, creating a continuous DNA strand.

Proofreading: DNA polymerase has proofreading capabilities and can correct errors during replication. This ensures high fidelity (accuracy) in DNA replication.

Example: Imagine a DNA strand with the sequence 3'-TACGATT-5'. The complementary template strand will be 5'-ATGCTAA-3'. A short RNA primer, say 5'-AUG-3', is added to the template strand. DNA polymerase adds nucleotides, extending the primer, resulting in something like: 5'-AUGCTAA-3'.

B. Transcription: Transcription is the process of copying the genetic information from DNA into RN

A. It takes place in the nucleus of eukaryotic cells.

Initiation: RNA polymerase binds to a specific region of DNA called the promoter. The promoter sequence signals the start of a gene.

Elongation: RNA polymerase unwinds the DNA double helix and synthesizes a complementary RNA molecule, called messenger RNA (mRNA), using one strand of the DNA as a template. RNA polymerase moves along the DNA template in the 3' to 5' direction, synthesizing the mRNA in the 5' to 3' direction. Uracil (U) replaces thymine (T) in RNA, so A pairs with

U. Termination: RNA polymerase reaches a termination signal, a specific sequence of DNA that signals the end of the gene. The mRNA molecule is released from the DNA template.

RNA Processing (Eukaryotes): In eukaryotes, the mRNA molecule undergoes processing before it can be translated.

This includes: Capping: A modified guanine nucleotide is added to the 5' end of the mRN

A. Splicing: Non-coding regions of the mRNA, called introns, are removed, and the coding regions, called exons, are joined together.

Polyadenylation: A poly(A) tail, a string of adenine nucleotides, is added to the 3' end of the mRN

A. Example: If a DNA template strand has the sequence 3'-TACGATT-5', the mRNA sequence transcribed from it will be 5'-AUGCUAA-3'. Note the replacement of T with

U. C.

Translation: Translation is the process of using the information encoded in mRNA to synthesize a protein. It takes place in the ribosomes in the cytoplasm.

Initiation: The mRNA molecule binds to a ribosome. A transfer RNA (tRNA) molecule carrying the amino acid methionine (Met) binds to the start codon (AUG) on the mRN

A. Elongation: The ribosome moves along the mRNA, one codon at a time. For each codon, a tRNA molecule carrying the corresponding amino acid binds to the mRNA. The amino acid is added to the growing polypeptide chain. Peptide bonds are formed between adjacent amino acids.

Termination: The ribosome reaches a stop codon (UAA, UAG, or UGA) on the mRNA. There is no tRNA molecule that corresponds to the stop codon. Release factors bind to the ribosome, causing the polypeptide chain to be released.