gene expression

• Gene expression involves the transcription and translation of DNA sequences. Only a fraction of genes in the cell are expressed.
• RNA is a single stranded and is composed of RNA nucleotides.
• RNA nucleotides consist of a phosphate group, ribose sugar and one of three nitrogenous bases. Adenine, Uracil, Guanine and Cytosine
• The three types of RNA are:

  • mRNA
  • tRNA
  • rRNA
• mRNA carries a complimentary copy of the DNA code from the nucleus to the ribosome. mRNA is transcribed from DNA in the nucleus and translated into proteins in the cytoplasm.
• Each triplet of bases on mRNA is known as a codon and codes for 1 specific amino acid
• Transfer RNA (tRNA) folds due to complementary base pairing. tRNA also has a triplet of bases exposed known as an anticodon at one end of the tRNA molecule.
• rRNA or ribosomal RNA, together with proteins, form the ribosome.
• RNA polymerase moves along DNA unwinding the double helix and breaking the hydrogen bonds between bases. RNA polymerase synthesises a primary transcript of mRNA from RNA nucleotides by complementary base paring.
• During RNA splicing regions known as introns, or non-coding regions, are removed from the primary transcript and the exons or coding regions remain and are spliced together.
• The exons when spliced together form a mature (mRNA) transcript. The order of the exons remains unchanged.
• TRANSCRIPTION:

  • The first stage of protein synthesis occurs in the nucleus.
  • mRNA is transcribed from DNA in the nucleus and translated into proteins by ribosomes in the cytoplasm.
• Introns are non-coding regions of DNA that are transcribed but not translated into proteins, while exons are coding regions of DNA that are transcribed and translated into proteins.
• The purpose of RNA splicing in gene expression is to remove introns and join together exons to produce a mature mRNA molecule that can be translated into a protein.
• Different proteins can be expressed from one gene as a result of alternative RNA splicing. Different mature mRNA transcripts are

  produced from the same primary transcript depending on which exons are retained.
• During translation, ribosomal subunits assemble together on the strand of mRNA, where they proceed to attract tRNA molecules tethred to amino acids. A long chain of amino acids emerges as the ribosome decodes the mRNA sequence into a polypeptide, or a new protein.
• tRNA is involved in the translation of mRNA into a polypeptide at a ribosome. Translation begins at a start codon and ends at a stop codon. Anticodons bond to codons by complementary base pairing, translating the genetic code into a sequence of amino acids. Peptide bonds join the amino acids together. Each tRNA then leaves the ribosome as the polypeptide is formed.
• Amino acids are linked by peptide bonds to form polypeptides. Polypeptide chains fold to form the three-dimensional shape of a protein, held together by hydrogen bonds and other interactions between individual amino acids. Proteins have a large variety of shapes which determines their functions.