protein synthesis

• Definition and importance of Messenger RNA (mRNA)

  -Made during transcription.

  -It carries the genetic code from the DNA to the ribosomes, where it is used to make a protein during translation.

  -A single polynucleotide strand.

  -Groups of three adjacent bases are usually called codons, or triplets.

  
• Definition and importance of Transfer RNA (tRNA)

  -Involved in translation.

  -Carries the amino acids that are used to make proteins to the ribosomes.

  -A single polynucleotide strand that is folded into a clover shape.

  -Hydrogen bonds between specific base pairs hold the molecule in this shape.

  -Every tRNA molecules has a specific sequence of bases at one end called an anticodon.

  -They also have an amino acid binding site at the other end.

  
• Describe the sequence of events during Transcription (Protein Synthesis)

  During transcription, an mRNA copy of a gene is made from DNA. In eukaryotic cells, transcription takes place in the nucleus (prokaryotic cells don't have a nucleus, so transcription takes place in the cytoplasm).

  
  

  1. Transcription starts when RNA polymerase (an enzyme) attaches to the DNA double helix at the beginning of a gene.

  2. The hydrogen bonds between the two DNA strands in the gene break, separating the strands and the DNA molecule uncoils at this point, exposing some of the bases.

  3. One of the strands is then used as a template to make an mRNA copy.

  4. The RNA polymerase lines up free RNA nucleotides alongside the exposed bases on the template strand. Free bases are attracted to the exposed bases. Specific complementary base pairing means that the mRNA strand ends up being a complementary copy of the DNA template strand (except the base T is replaced by U in RNA).

  5. Once the RNA nucleotides have paired up with their specific bases on the DNA strand, they are joined together by RNA polymerase, forming an mRNA molecule.

  6. The RNA polymerase moves along the DNA, separating the strands and assembling the mRNA strand.

  7. The hydrogen bonds between the uncoiled strands of DNA reform once RNA polymerase has passed by and the strands coil back into a double helix.

  8. When RNA polymerase reaches a particular sequence of DNA called a stop signal, it stops making mRNA and detaches from the DNA.

  9. In eukaryotes, mRNA moves out of the nucleus through a nuclear pore and attaches to a ribosome in the cytoplasm, where the next stage of protein synthesis takes place.

  
• Products of Transcription


  In eukaryotes, the introns and exons are both copied into the mRNA during transcription. mRNA strands containing introns and exons are called pre-mRNA. A process called splicing then occurs - introns are removed and the exons are joined together, forming mRNA strands. This takes place in the nucleus.

  
  

  The mRNA then leaves the nucleus for the next stage of protein synthesis - translation.

  
  

  In prokaryotes, mRNA is produced directly from the DNA - without splicing taking place (there is no need for splicing because there are no introns in prokaryotic DNA).
• Describe the sequence of events during Translation (protein synthesis)

  In both eukaryotes and prokaryotes, translation occurs at the ribosome in the cytoplasm. During translation, amino acids are joined together to make a polypeptide chain (protein), following the sequence of codons (triplets), carried by the mRNA.

  
  

  1. The mRNA attaches itself to a ribosome and transfer RNA (tRNA) molecules carry amino acids to it. ATP provides the energy needed for the bond between amino acid and the tRNA molecule to form.

  2. A tRNA molecule (carrying an amino acid) with an anticodon that is complementary to the first codon on the mRNA attaches itself to the ribosome by specific base pairing.

  3. A second tRNA molecule attaches itself to the next codon on the mRNA in the same way.

  4. The two amino acids attached to the tRNA molecules are joined by a peptide bond. The first tRNA molecule moves away, leaving its amino acid behind.

  5. A third tRNA molecule binds to the next codon on the mRNA. Its amino acid binds to the first two and the second tRNA molecule moves away.

  6. This process continues, producing a chain of linked amino acids (a polypeptide chain) until there is a stop signal on the mRNA molecule.

  7. The polypeptide chain moves away from the ribosome and translation is complete.
• Explain The Genetic Code

  -The genetic code is a sequence of base triplets (codons) in mRNA which code for specific amino acids.

  -In the genetic code, each base triplet is read in sequence, separated from the triplet before it and after it. Triplets do not share bases as the code is non-overlapping.

  -The genetic code is also degenerate as there are more possible combinations of triplet than there are amino acids (20 amino acids and 64 possible combinations of triplet). This means that some amino acids care coded for by more than one base triplet e.g. tyrosine can be coded for by UAU or UAC.

  -Some triplets are used to tell the cell when to start and stop production of the protein. These are called start and stop codons. They are found at the beginning and end of mRNA. For example, AUG is a start codon and UAG is a stop codon.

  -The genetic code is also universal as the same specific base triplets code for the same amino acids in all living things. For example, UAU codes for tyrosine in all organisms.
• Explain Homologous Pairs


  -Humans have 46 chromosomes in total.

  -23 pairs.

  -One chromosome in each pair came from the mother and one came from the father.

  -The chromosomes that make up each pair are the same size and have the same genes, although they could have different versions of those genes called alleles.

  -These pairs of chromosomes are called homologous pairs.