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Flashcards in 3. Chromosomes Deck (18):

Contrast the DNA in prokaryotic and eukaryotic cells.

- contain a linear DNA molecule
- associated with histone proteins
- no plasmids
- two or more different chromosomes

- 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.



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


What happens to a sister chromatid at the start of anaphase?

the centromere divides allowing the chromatids to become separate chromosomes


Define 'genome'. How big is the genome in terms of chromosomes? What units is the genome measured in?

- the whole of the genetic information of an organism
- size of genome = total amount of DNA in one set of chromosomes in that species
- millions of base pairs (bp) of DNA


Give 5 examples of organism and the size of their genome (measured in millions of bp).

1. T2 phage - 0.18
2. Escherichia coli - 5
3. D. Melanogaster - 140
4. Homo sapiens - 3,000
5. Paris japonica - 150,000

T2 phage = virus that attacks E.Coli
Paris japonica = woodland plant with a remarkably large genome size

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