Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

This page supports the lesson note with a companion video and a short classroom-ready summary.

For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.

Performance objectives

• Describe the process of DNA replication.
• Explain the process of transcription.
• Describe the process of translation.
• Explain the central dogma of molecular biology.
• Relate the structure of DNA to its function.

Lesson summary

This week, we delve deeper into the fascinating world of DNA, the molecule that carries the genetic blueprint for all living organisms. We'll focus on the core principles of DNA replication, transcription, and translation – the processes by which genetic information is copied and used to create proteins. Understanding these processes is crucial for comprehending inheritance, genetic diseases, and the very basis of life itself. In the South African context, this knowledge is particularly relevant to understanding the genetic predispositions to certain diseases prevalent in our population, such as diabetes, heart disease, and certain types of cancer.

Lesson notes

2.1 DNA Replication: Copying the Code DNA replication is the process by which a DNA molecule is duplicated, ensuring that each daughter cell receives a complete set of genetic information during cell division. It's a semi-conservative process, meaning that each new DNA molecule consists of one original (template) strand and one newly synthesized strand.

Steps of DNA Replication: Initiation: Replication begins at specific sites on the DNA molecule called origins of replication. The enzyme helicase unwinds and separates the two DNA strands, creating a replication fork.

Elongation: DNA polymerase is the key enzyme responsible for synthesizing new DNA strands. It can only add nucleotides to the 3' end of an existing strand, meaning DNA synthesis always proceeds in the 5' to 3' direction. Since DNA strands are antiparallel, replication occurs differently on each strand: Leading strand: Synthesized continuously towards the replication fork. DNA polymerase can move along this strand adding nucleotides as the replication fork opens.

Lagging strand: Synthesized discontinuously in short fragments called Okazaki fragments. This is because the DNA polymerase can only add nucleotides to the 3' end, and the replication fork is moving in the opposite direction. The synthesis of each Okazaki fragment requires an RNA primer to initiate the process. Primase synthesises RNA primers, which are then replaced with DN

A. Termination: Replication continues until the entire DNA molecule is copied. Once completed, the enzyme DNA ligase joins the Okazaki fragments together to form a continuous strand. The RNA primers are replaced with DNA nucleotides before ligation.

Key Enzymes in DNA Replication: Helicase: Unwinds and separates the DNA strands.

DNA polymerase: Adds nucleotides to the growing DNA strand. Has proofreading capabilities.

Primase: Synthesizes RNA primers to initiate DNA synthesis on the lagging strand.

Ligase: Joins Okazaki fragments together.

Exonucleases: Enzymes that remove nucleotides from the ends of DNA strands, used to remove RNA primers.

Example: Imagine you have a short sequence of DNA that needs to be replicated: 5'-ATGC-3' on one strand, which is paired with 3'-TACG-5' on the other strand. Helicase will first separate the strands. On the leading strand (3'-TACG-5'), DNA polymerase will continuously add complementary nucleotides, moving along the strand towards the replication fork. On the lagging strand (5'-ATGC-3'), primase will synthesize short RNA primers. Then DNA polymerase adds nucleotides to the 3' end of each RNA primer to create Okazaki fragments. Finally, DNA ligase will join the Okazaki fragments to create a continuous strand. 2.2 Transcription: From DNA to RNA Transcription is the process of synthesizing RNA from a DNA template. This process occurs in the nucleus.

Steps of Transcription: Initiation: RNA polymerase binds to a specific region of the DNA called the promoter. The promoter signals the start of the gene. RNA polymerase then unwinds the DNA double helix at the promoter region.

Elongation: RNA polymerase moves along the DNA template strand, adding complementary RNA nucleotides to the growing RNA molecule. The RNA transcript is synthesized in the 5' to 3' direction, using the DNA template strand as a guide. Note that in RNA, uracil (U) replaces thymine (T) as a base. So, where there is an adenine (A) on the DNA template strand, a uracil (U) will be added to the RNA transcript.

Termination: RNA polymerase reaches a termination signal on the DNA template. The RNA transcript is released, and RNA polymerase detaches from the DN

A. Key Components of Transcription: RNA polymerase: The enzyme that synthesizes RN

A. Promoter: A specific DNA sequence that signals the start of a gene and is recognized by RNA polymerase.

Template strand: The DNA strand used as a template for RNA synthesis.

RNA transcript: The newly synthesized RNA molecule.

Example: Consider a DNA template strand with the sequence 3'-TACGATT-5'. During transcription, RNA polymerase will bind to the promoter region upstream of this sequence. The RNA polymerase will then move along the DNA template strand synthesizing an RNA transcript with the sequence 5'-AUGAAUA-3'. Note how uracil (U) replaces thymine (T). 2.3 Translation: From RNA to Protein Translation is the process of synthesizing a protein from an mRNA template. This process occurs in the ribosomes in the cytoplasm.

Steps of Translation: Initiation: The mRNA molecule binds to a ribosome. A 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, reading the codons (three-nucleotide sequences) one at a time. For each codon, a tRNA molecule with the corresponding anticodon (complementary to the mRNA codon) brings the appropriate amino acid to the ribosome. The amino acids are joined together by peptide bonds, forming a polypeptide chain.