Chromosomes (3.2) Flashcards
Prokaryote genetic material
A single chromosome consisting of a circular DNA molecule (genophore). May possess additional genetic material in plasmids. DNA is naked - not associated with proteins.
Eukaryote genetic material
Homologous chromosomes. One of maternal origin and one of paternal origin. DNA is packaged with histone proteins.
Describe the packaging of DNA in eukaryotic cells.
DNA is wound around eight histone proteins to form a nucleosome. Nucleosomes are linked by an additional histone protein to form a string of chromatosomes. These then coil to form a solenoid structure which is condensed to form a 30 nm fibre. These fibres then form loops, which are compressed and folded around a protein scaffold to form chromatin. Chromatin will then supercoil during cell division to form chromosomes that are visible (when stained) under a microscope.
Define diploid.
Nuclei possessing pairs of homologous chromosomes (2n). Two alleles for each trait. Sexually reproducing organisms are diploid.
Define haploid.
Nuclei possessing only one set of chromosomes (n). Single allele for each trait.
What are the sex chromosomes in humans?
Females (XX)
Males (XY)
What is a karyogram?
A diagram or photograph of the chromosomes in a cell, arranged in homologous pairs and in a numbered sequence. A visual profile.
What is a karyotype?
An individual’s complete set of chromosomes.
Process of a karyotype.
- A sample of cells is harvested. Usually from a foetus or white blood cells of adults.
- The cells are chemically induced to undergo mitosis and stop at metaphase.
- The chromosomes are stained and spread out on a slide, then photographed.
- The photograph is cut up and the chromosomes are arranged in homologous pairs according to size, shape, banding patterns and the position of the centromere.
What is karyotyping used for?
Pre-natal diagnosis of chromosomal abnormalities eg. downs syndrome. It’s also used to determine the gender of the baby (via identification of the sex chromosomes).
What is Downs Syndrome?
A condition due to a mutation where the individual has an additional copy of chromosome 21, i.e. three rather than two.
Cairns’ technique
Used to measure the length of DNA molecules by autoradiography.
John Cairns incubated bacterial cells in a solution containing radioactive thymidine. The solution was incorporated into the chromosomal DNA of the cell. The chromosomes were isolated from the cell and fixed to a photographic surface where they were immersed in silver bromide. When the photographic film developed the chromosomal DNA was able to be visualised with an electron microscope.
