Lessons 28–29 · 2 lessons · A. G‘afurov, A. Abdukarimov, J. Tolipova, O. Ishankulov, M. Umaraliyeva, I. Abdurahmonova. Biology, Grade 10, 1st edition. “Sharq” publishing and printing joint-stock company, Tashkent, 2017
28
Genetic elements of the cell
Textbook: pp. 123–125
GoalExplain the genetic elements of prokaryotic and eukaryotic cells (nucleoid, chromosome, plasmids, transposons).
In all living forms hereditary information is stored in nucleic acid: DNA or RNA in viruses, a circular DNA in the cytoplasm – the nucleoid – in prokaryotes (bacteria), and the nuclear chromosomes (DNA + histone proteins) in eukaryotes. Bacterial DNA does not form a complex with histones, so its genes are constantly “on”. Plasmids are circular double-stranded DNA molecules tens to hundreds of times smaller than the main chromosome, carrying relatively few genes. Transmissible plasmids can join the bacterial chromosome, while autonomous plasmids replicate independently and can pass from one cell to another. Plasmids often carry antibiotic-resistance genes, so resistance spreads fast among microbes; in genetic engineering plasmids serve as vectors – carriers of a foreign gene. Transposons (mobile elements) are DNA pieces able to change position in the genome; they were discovered in maize by B. McClintock in the 1940s (Nobel Prize, 1983). Retrotransposons move through an RNA copy (with reverse transcriptase) and increase the amount of DNA.
Worked examples
If a resistance gene for an antibiotic lies on a plasmid, any bacterium that receives the plasmid becomes resistant too. This is one reason antibiotics should be taken only when needed and as prescribed by a doctor.
If a transposon enters the red-pigment gene in a maize grain, no pigment forms (colourless grain); when the transposon moves out, the cell makes pigment again and coloured spots appear on the grain.
Class activity
“Table of genetic elements”: groups fill in a table of location and function for nucleoid, plasmid, transposon and chromosome.
Practice
1
Compare the genetic apparatus of prokaryotes and eukaryotes.
Prokaryotes: circular DNA (nucleoid) in the cytoplasm, without histones; eukaryotes: chromosomes in the nucleus, DNA with histone proteins.
2
What is the difference between transmissible and autonomous plasmids?
A transmissible plasmid joins the chromosome and loses independence; an autonomous plasmid replicates independently.
3
A plasmid is 100 times smaller than the chromosome. If the chromosome is 4 600 000 base pairs, about how many base pairs is the plasmid? (Number only.)
46000
4
Why are plasmids convenient vectors in genetic engineering?
They are small, circular, replicate independently, can pass between bacteria, and a foreign gene is easily inserted.