Replication and Transcription: Copying and Reading the Gene: Quick Revision

A brief note on Replication and Transcription: Copying and Reading the Gene. Read it before a test. Try each question at the end before you open its answer.

Replication

• Semiconservative: every new double helix has one old strand and one new strand.
• Meselson and Stahl (1958): E. coli in 15N, then in 14N, spun in caesium chloride. After 20 minutes, one hybrid band. After 40 minutes, a hybrid band and a light band, in equal amounts.
• After $n$ generations, the hybrid share is $2 \div 2^n$. Taylor showed the same for chromosomes, with Vicia faba.
• Origin (ori): replication starts there and goes both ways, with two forks. E. coli is copied in about 18 minutes.
• DNA-dependent DNA polymerase: uses a template and the dNTPs, and adds only to a 3′ end, so a strand grows 5′ to 3′.
• Leading strand: continuous. Lagging strand: Okazaki fragments, joined by DNA ligase.

Transcription

• Central dogma: DNA to RNA to protein. Reverse transcription goes from RNA to DNA.
• Transcription unit: promoter, structural gene, terminator. The template strand is read 3′ to 5′. The RNA grows 5′ to 3′, with U for T, and has the sequence of the coding strand.
• Bacteria: one RNA polymerase, sigma to start and rho to stop, and no processing.
• Eukaryotes: RNA polymerase I (rRNAs), II (hnRNA) and III (tRNA, 5S rRNA, snRNAs).
• hnRNA: splicing (introns out, exons joined), capping (methyl guanosine triphosphate at the 5′ end) and tailing (200 to 300 adenylate residues at the 3′ end).

A worked example

A fork copies 9000 nucleotides of the lagging strand in fragments of 1500 nucleotides. How many fragments, and how many joints?

1. $9000 \div 1500 = 6$ fragments.
2. There is one joint fewer than there are fragments, so $5$ joints.

Practice with new numbers

A program to try

The program shows the share of hybrid DNA after some generations.

Mistakes to avoid

• Saying that 15N is radioactive. It is only heavy.
• Saying that the RNA is complementary to the coding strand. It is the same as the coding strand, with U for T.
• Saying that the polymerase adds to the 5′ end. It adds to the 3′ end.
• Leaving the introns in the mRNA. Splicing removes them.

Check yourself

1. What does the tube show after two generations in the 14N medium?

2. Which strand is made in fragments, and which enzyme joins them?

3. Which strand of the gene has the same sequence as the RNA, apart from U for T?

4. Name the three steps of the processing of the hnRNA.

Sources

• NCERT (2023). Biology, Textbook for Class XII, chapter 5, Molecular Basis of Inheritance.
• Clark M. A. et al. (2018). Biology 2e, OpenStax, chapters 14 and 15, DNA Structure and Function, Genes and Proteins.