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Chapter 8.3 DNA and Chromosomes
Differences between prokaryotic and eukaryotic cells extend to their DNA:
In prokaryotic cells, the DNA molecules are smaller, form a circle and are not
associated with protein molecules therefore they do not have chromosomes.
In eukaryotic cells, the DNA molecules are larger, are linear and occur in association
with proteins to form chromosomes.
Chromosomes are only visible as distinct structures during cell division. For the rest of the time,
they are widely dispersed throughout the nucleus. When visible, they appear as two threads,
joined at a single point. Each thread is called a chromatid. The DNA in chromosomes is held in
position by proteins. The considerable length of DNA found in each cell is highly coiled and
folded. DNA is a double helix. The helix is wound around proteins to sustain a fixed position.
The DNAprotein complex is then coiled. The coil is looped and further coiled before being
packed into the chromosome. This allows a lot of DNA to be condensed into a single
chromosome. In almost all species, there is an even number of chromosomes in the cells of
adults. This is because chromosomes occur in homologous pairs.
Sexually produced organisms are the result of the fusion of a sperm and an egg, each of which
contributes a set of chromosomes to the offspring. Therefore, one of each is derived from the
chromosomes provided by the maternal chromosomes and the other is derived from the
paternal chromosomes. These are known as homologous pairs, and the total number is
referred to as the diploid number. In humans, this is 46. A homologous pair is always two
chromosomes that determine the same genetic characteristics, but determining the same
genetic characteristics is not the same as being identical. During meiosis, the halving of the
number of chromosomes is done in a manner which ensures that each daughter cell receives
one chromosome from each homologous pair. When these haploid cells combine, the diploid
state is restored.
What is an allele?
Each gene exists in two, occasionally more, different forms. Each of these forms is an allele.
Each individual inherits one allele from each of its parents. These two alleles can be the same
or different. When they are different, each allele will code for a different polypeptide.