B3 - Molecules of Life

B3  - Molecules of life

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Cell Structure

Molecules of life - Cell Structure

The number of mitochondria in the cytoplasm depends on the activity of the cell.

Respiration occurs in the mitochondria.

Liver or muscle cells have a lot of mitochondria - carry out many functions & the mucle contracts. They both need lots of energy.

Ribosomes are smaller than mitochondria - found in cytoplasm. Too small to be seen with a light microscope & are the site of protein synthesis.

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DNA and the genetic code

  • The nucleus contains genes. Each gene:

- is a section of a chromosome made of DNA

- codes for a particular protein

  • DNA is made of two strands coiled to form a double helix, each strand contains chemicals called bases. There are four different types of bases, with cross links between the strands formed by pairs of bases. In every gene there is a different sequence of bases.
  • Proteins are made in the cytoplasm but DNA can't leave the nucleus. So a copy of the gene has to be made that can leave the nucleus and carry the code to the cytoplasm.
  • The four bases in DNA are called A,C,T and G. The cross links holding the two strands together are always between the same bases, A-T, C-G. < Complemenatry base pairing.
  • DNA base code controls which protein is made. This is because the base sequence in the dna codes for the amino acid sequence in the protein. Each amino acid is coded for by a sequence of three bases. 
  • The code needed to carry the protein to the DNA is called messenger RNA or mRNA.
  • Many of the proteins that are made are enzymes, which can control the activity of the cell.
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Discovering the structure of DNA

  • Watson and Crick built amodel of DNA using data from other scientists. Two important peices of data they used were:

-  photos taken using an x-ray which showed that DNA had two chains wound in a helix

- data indicating that the bases occured in pairs.

  • Watson and Crick worked out the structure of DNA  in 1953 and shared the nobel prize for it in 1962. There is a delay because other scientists need to make sure that it is correct.
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