DNA and protein synthesis (pg. 53-55)

• The nucleus contains your genetic material in the form of chromosomes.
• Chromosomes are long lengths of DNA coiled up.
• A gene is a short section of DNA.
• DNA is a long list of instructions on how to put an organism together and make it work.
• Human body cells are diploid: this means they have two copies of each chromosome, arranged in pairs. A human cell nucleus contains 46 chromosomes in total, so the diploid number for a human is 46.
• All of an organism's DNA makes up the organism's genome.
• Each separate gene in a DNA molecule is a chemical instruction that codes for a particular protein.
• Proteins control most processes in the body and determine inherited characteristics.
• There can be different versions of the same gene, which gives different versions of a characterstic like blue or brown eyes.
• Alleles are different versions of the same gene.
• A DNA molecule has two strands coiled together in the shape of a double helix (two spirals). The two strands are held together by two chemicals called bases.
• There are 4 different bases in a DNA molecule:

  1. adenine (A).
  2. thymine (T).
  3. cytosine (C).
  4. guanine (G).
• The bases in a DNA molecule are paired, and they always pair up in the same way - it's always A-T and C-G. This is called complementary base-pairing.
• DNA controls the production of proteins (protein synthesis) in a cell.
• Each different protein has its own particular number and order of amino acids. The amino acid chains fold up to give each protein a different, specific shape, which means each protein can have a different function.
• The order of the bases in a gene determines the order of amino acids in a protein.
• Each amino acid is coded for by a sequence of three bases in the gene (called a codon or a base triplet).
• DNA contains four different bases and each codon in a gene contains three bases. So there are 4 x 4 x 4 = 64 possible codons. Since there are only 20 amino acids, some codons code for the same amino acid.
• Many regions of DNA are non-coding, which means that they don't code for any amino acids. However, some are still involved in protein synthesis.
• Proteins are made in two stages:

  1. Transcription.
  2. Translation.
• Proteins are made in the cell cytoplasm by subcellular structures called ribosomes.
• DNA is found in the cell nucleus and can't move out of it because the nucleus is really big.
• The molecule messenger RNA (mRNA) gets the information from the DNA to the ribosome in the cytoplasm.
• Like DNA, mRNA is made up of a sequence of bases, but it's shorter and only a single strand. It also uses uracil (U) instead of thymine (T) as a base.
• RNA polymerase is the enzyme involved in joining together the base sequence to make mRNA.
• Transcription:

  1. RNA polymerase binds to a region of non-coding DNA in front of a gene.
  2. The two DNA stands unzip and the RNA polymerase moves along one of the strands of DNA.
  3. It uses the coding DNA in the gene as a template to make the mRNA. Base pairing between the DNA and RNA ensures that the mRNA is complementary to the gene.
  4. Once made, the mRNA molecule moves out of the nucleus and joins with a ribosome in the cytoplasm.
• Translation:

  1. Amino acids are brought to the ribosome by another RNA molecule called transfer RNA (tRNA).
  2. The order in which the amino acids are brought to the ribosome matches the order of the codons in mRNA.
  3. Part of the tRNA's structure is called an anticodon - it is complementary to the codon for the amino acid. The pairing of the codon and anticodon makes sure that the amino acids are brought to the ribosome in the correct order.
  4. The amino acids are joined together by the ribosome. This makes a protein.