Written by amberstudies

Life Sciences: Chromosomes

Hi

So this is the beginning of the ‘chromosomes and meiosis’ section

WHAT ARE CHROMOSOMES?

  • They are long & thread-like structures that form a part of the chromatin network within the nuclei of cells
  • They are made of a strand of DNA that is wound around a protein core called a histone
  • They were discovered by chance in 1888
  • They absorb colour dye very easily, thus names chromosomes ‘coloured bodies’
  • Visible under a microscope but as individual threads whilst the cell is dividing
  • Found in somatic cells of diploid organisms
  • The number of chromosomes in every cell is exactly the same
  • They are made up of two sets- maternal and paternal and are thus known as diploid cells (2n)
  • For every maternal chromosome there is a paternal chromosome- this means that they are a homologous pair. These chromosomes forming a pair are the same shape and size but the alleles for each trait are not the same
  • The DNA of each chromosome replicates to form 2 identical threads or chromatids joined by a centromere- this occurs in the interphase of a cell cycle (in between cell divivsions). These threads are only visible when they shorten and thicken as a cell is dividing.
  • This replication is so important because it ensures that the daughter cells have the identical genetic code as the parent cells

WHAT IS A KARYOTYPE?

Its is a set of chromosomes. It shows the number, size and shape of the chromosomes during metaphase of mitosis. In the human karyotype that shows the non-sex chromosomes (autosomes) of a similar size grouped and placed in groups A-G according to their length. The sex chromosomes (gonosomes) which are X-female and Y-male, are placed separately.

WHAT DO THEY SHOW?

  • Whether a cell comes from a male or female
  • Abnormal chromosomes

WHAT IS MEANT BY A CHROMOSOME NUMBER?

Each species has a specific number of chromosomes in its somatic cells (body cells). It is possible that some of them have the same amount without being related or the same; it is the similarities in the DNA of the chromosomes rather that shows the relationships, not the number of chromosomes.

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