3. Chromosomes Flashcards
Contrast the DNA in prokaryotic and eukaryotic cells.
Eukaryotic:
- contain a linear DNA molecule
- associated with histone proteins
- no plasmids
- two or more different chromosomes
Prokaryotic:
- consists of a circular DNA molecule
- naked - no associated proteins
- plasmids often present
- one chromosome only
How can we know where substances are located in cells?
- using autoradiography combined with electron microscopy
- finds radioactively labelled substances in a cell
Describe the process of autoradiography.
- radioactively label substance you want to locate
- coat thin sections of cells with a photographic film
- leave in darkness for days or weeks
- develop the film that is coating the section of cells
- view film through microscope, you will see black dots where a radioactive atom has decayed and given out radiation (acting like light on the film) - taken together, the dots show the structure
What did John Cairns do with autoradiography?
research the chromosome of E.coli, a prokaryote
Who researched the chromosome of E.coli, a prokaryote, using autoradiography?
John Cairns
What did John Cairns discover?
that the DNA molecules from E.Coli cells were circular and 1,100um long, despite the E.Coli cells only being 2um long
How did John Cairns make his discovery?
using autoradiography:
- grew E.Coli in a medium containing radioactively labelled thymine, so its DNA became labelled but not RNA
- placed cells on a membrane and digested their cell walls, allowing the DNA to spill out over the membrane
- coated the membrane with a photographic film and left it in the dark for two months
- developed film: lines of black dots showed the position of the DNA molecules from E.Coli
Other researchers (not John Cairns) used similar techniques to him, but investigated eukaryote chromosomes. What did they discover (general)?
- eukaryote chromosomes contained linear rather than circular DNA
- DNA molecules were much longer
Give an example of an eukaryotic organism for which researchers used autoradiography to find the length of its chromosome? What was the length of the chromosome in this organism? What does this length show?
- fruit fly (D. Melanogaster)
- 12,000um long (12 mil)
- shows that eukaryote chromosomes contain one very long DNA molecule rather than a number of shorter molecules
When are eukaryote chromosomes visible with a light microscope?
only easily visible during mitosis
What happens to eukaryote chromosomes in prophase? What happens to eukaryote chromosomes in metaphase?
- they condense
- metaphase: reach their minimum length
You should be able to recognise micrographs of chromosomes in the various stages of mitosis.
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What does each chromosome in prophase and metaphase consist of?
two structures - sister chromatids
In prophase and metaphase, each chromosome consists of two structures called sister chromatids. What do they each contain?
a DNA molecule that was produced by replication during interphase (so their base sequences are identical)
How are sister chromatids held together?
by a centromere
