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Chromosome Structure:
Chromosomes are only visible as distinct structures when a cell is dividing
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When they first become visible the chromosomes appear as two
threads, joined at a single point
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The DNA in chromosomes is held by histones
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DNA and chromosomes

Homologous Chromosomes:

Humans are the result of the fusion of a sperm and egg, each of which contributes one complete
set of chromosomes to offspring
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These are known as homologous pairs and the total number is referred to as the diploid number
(in humans its 46)
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For instance, a homologous pair of chromosomes may each possess genes for tongue rolling and
blood group, but one chromosome may carry the allele for non-roller and blood group A whereas
the other carries the allele for roller and blood group B
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In this way, each cell receives one gene foe each characteristic of the organism
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What is an Allele:
An allele is one of a number of alternative forms of a gene
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This DNAHistone complex is then coiled which is
then looped and further coiled before
being packed into the chromosome
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Humans have 46 chromosomes (23
pairs)
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Genes are sections of DNA that contain coded information in the form of specific
sequences of bases
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Each of these forms is an allele
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These two alleles may be
the same or they may be different
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Any changes in the base sequence of a gene produces a new allele of that gene (a
mutation) and results in a different sequence of amino acids being coded for
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Sometimes this different protein may not function properly or at
all, causing consequences for the organism

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