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When an organism develops from a single celled zygote into a multicellular adult its cells become differentiated into many cell types. The molecular mechanism which is thought to underlie this pr...ocess of cytodifferentiation is thought to be differential gene transcription, whereby different genes in the inherited chromosomal DNA are transcribed - that is, different parts of the genetic code are read to produce different cell types. In this programme an attempt is made to test directly this hypothesis that cytodifferentiation is due to differential gene transcription. To do this the giant chromosomes of the salivary gland of a fly maggot are used and an explanation of their 'giant' structure follows their isolation from the salivary glands of a living maggot. The giant chromosomes have a regular ladder-like, banded structure, This can be seen at times to be lost where 'rungs' balloon out into structures known as 'puffs' These chromosome puffs are shown to be the sites of gene transcription. It is then shown - (1) that the chromosomes of different cell types have their own specific banding and puffing patterns along their lengths (2) Cell differentiation is a progressive process; it proceeds with time and it is shown by comparing the banding and puffing patterns of particular salivary gland chromosorues that different genes are transcribed or 'switched on' at different times. These two demonstrations are strong evidence in favour of the hypothesis that cell differentiation is the result of differential gene transcription.
Metadata describing this Open University video programme
Item code: S2-5; 03
First transmission date: 19-08-1973
Published: 1973
Rights Statement:
Restrictions on use:
Duration: 00:23:58
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Producer: Roger Jones
Contributor: Norman Cohen
Publisher: BBC Open University
Keyword(s): Banding; Chromosomes; Cytodifferentiation; DNA; Molecular mechanism; Puffs; Rungs; Transcribed; Zygote
Footage description: Norman Cohen introduces the programme by giving a brief outline: differential gene transcriptior as the mechanism underlying cyto-differentiation Cohen describes the differential gene transcription hypothesis with the aid of a model of the DNA helix. Time-lapse shots showing cellular mitosis Cohen discusses the problem of observing chromosomes during interphase when gene transcription (RNA synthesis) takes place. Cohen explains why cells from the salivary glands of drosophila larvae are used for chromosomal studies, drosophila in a jar. He shows some Close up shot (under magnification). Cohen uses a plastic model of drosophila larvae to show how salivary glands are exposed during dissection of a specimen. Shot of a salivary gland cell of a drosophila larva. Chromosomes are clearly visible. Cohen demonstrates the technique of dissecting a drosophila larva to obtain a salivary gland speciment. He prepares this for microscopic analysis. Shot of prepared slide under magnification (after staining.) The Chromosomes are clearly seen. Cohen uses chromosome and DNA models to explain why chromosomes are visible in the non-dividing cells of insect salivary glands whereas they are visible in other cells only during mitosis. Cohen shows how 'giant' chromosomes are built up in diosophila larvae salivary glands. At the same time Cohen explains how 'banding' occurs in chromosomes. Shot of chromosomes showing banding. Cohen begins examination of Chromosome 'puffs'. Shot of chromosome under magnification showing 'banding' and 'puffs'. Cohen uses a 'giant' chromosome model to demonstrate the 'puffing' mechanism in chromosomes. He explains what is happening - RNA synthesis. Shot of a chromosome taken by a technique called auto-radiography. This provides experimental verification of RNA synthesis in chromosome 'puffs'. Cohen explains. Cohen with diagrams of chromosomes from 4 different tissues. He compares the 'banding' and 'puffing' patterns. Cohen with photograph of 4 different chromosome preparations from disophila salivary glands. He compares the 'banding' and 'puffing' patterns of these preparations and explains how these can show gene transcription over a period of time (15 hours in this case).
Master spool number: 6HT/70885
Production number: 00525_1021
Videofinder number: 2823
Available to public: no