Chromosomes

Chromosomes AS level biology Unit 2

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  • Created by: LZ95
  • Created on: 05-04-13 10:03

Chromosomes Structure

Structure:

-They are only visible as distinct structures when the cell is dividing.

 -When visible, chromosomes appear as two threads, joined at a point called the centromere. Each thread is called a chromatid.

-DNA in chromosomes is held together by proteins.

-The DNA helix is looped and coiled so that it can become more compact and this means a lot more DNA can 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 pairs called homologous pairs.

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Homologous chromosomes

  • One of each pair is derived from the chromosomes provided by the mother in the egg (maternal chromosomes) and the other is derived from the chromosomes provided by the father in the sperm (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 of chromosomes always determine the same characteristics; however, this does not mean they are identical i.e. one could code for blue eyes and blood group A whereas the other could code for brown eyes and blood group B.
  • During meiosis, each daughter cell receives one chromosome from each homologous pair. In this way each cell receives one set of information for each characteristic of the organism.
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Alleles

-Each gene exists in two different forms. Each of these forms is called an allele.

-Each individual will inherit one allele from each of its parents. These may be the same or they may be different.

-If different, they each code for a different polypeptide.

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