Section A: DNA, RNA and protein synthesis

• What is a gene?

  A gene is a section of DNA that contains the coded information for making polypeptides and functional RNA
• How is DNA stored in eukaryotic cells?

  Eukaryotic cells contain linear DNA that exist as chromosomes which are found in the nucleus. Eukaryotic DNA is also associated with histones to help support the DNA and help pack DNA tightly into chromosomes
• Describe the DNA of mitochondria and chloroplasts

  Have their own DNA which is similar to prokaryotic DNA as it's not associated with histones
• Describe the DNA in prokaryotic cells

  Prokaryotes carry DNA as chromosomes but the DNA molecules are shorter and circular and it condenses to fit in the cell by supercoiling.

  
• What determines the order of amino acids in a particular polypeptide?

  The sequence of nucleotides in DNA.
• How is one amino acid coded?

  By a specific sequence of three nucleotides called a triplet code or codon
• What do genes that don't code for a polypeptide code for?

  Functional RNA e.g tRNA and rRNA
• What is the complete set of genes in a cell known as?

  Genome
• What is the full range of proteins a cell is able to produce called?

  Proteome
• What are sections in genes that don't code for polypeptides?

  Introns
• What happens to introns during protein synthesis?

  Removed by a process called 'splicing' by the enzyme DNA ligase
• What are alleles?

  Genes that exist in more than one form and code for different versions of the same polypeptides
• How many pairs of chromosomes do humans have?

  23 pairs for a total of 46 chromosomes
• What are homologous chromosomes?

  Homologous chromosomes are chromosomes that have the same size and the same genes but have different alleles
• What is protein synthesis?

  Protein synthesis is the production of proteins from the information contained within a cell's DNA and has 2 stages:

  Transcription- Where the DNA code is copied into a molecule called mRNA

  Translation - where the mRNA joins with an organelle called a ribosome and the code it carries is used to synthesise a protein
• What is RNA?

  RNA (ribonucleic acid) is a single polynucleotide strand and it contains the nitrogenous base Uracil (U) as a base instead of thymine. Uracil always pairs with adenine during protein synthesis.
• What is mRNA?

  mRNA (messenger RNA) is a single polynucleotide strand that carries the genetic code from the DNA to the ribosomes, where it's used to make a protein during translation

  
• What is tRNA?

  Transfer RNA is a clover shaped single polynucleotide strand that carries the amino acids that are used to make proteins in the ribosomes. Hydrogen bonds between specific base pairs hold the molecule in this shape. Every tRNA molecule has a specific sequence of 3 bases at one end called an anticodon. Also involved in translation.

  
• Describe Step 1 of Transcription

  Transcription starts when RNA polymerase attaches to the DNA double helix at the beginning of a gene. In eukaryotes, the hydrogen bonds between the 2 DNA strands in the gene are broken by the enzyme DNA helicase . Seperating the strands, and the DNA molecule uncoils at that point, exposing some of the bases and one of the strands is then used as a template to make an mRNA copy

  
• Describe Step 2 of Transcription

  The RNA polymerase lines up free RNA nucleotides alongside the exposed bases on the template strand. The free bases are attracted to the exposed bases by complementary base pairing meaning mRNA strand ends up being a complementary copy of the DNA template strand except thymine is replaced by Uracil. Once the RNA nucleotides have paired up their specific bases on the DNA strand as they're joined together by RNA polymerase.

  
• Describe step 3 of Transcription

  The RNA polymerase moves alongside the DNA, assemvling the mRNA strand. The hydrogen bonds between the uncoiled strands of DNA reforms once the RNA polymerase has passed by and the strands coil back into a double helix

  
• Describe step 4 of transcription

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

  In eukaryotes, mRNA moves out of the nucleus through a nuclear pore and attaches to a ribosome in the cytoplasm, where the next stages of protein synthesis takes place
• How is mRNA edited in eukaryotic cells?

  Introns and exons are both copied into mRNA during transcription. mRNA strands containing introns and exons are called pre mRNA. SPlicing then occurs i.e introns are removed and the exons join together forming mRNA strands. This process takes place in the nucleus
• How is mRNA edited in prokaryotes?

  mRNA is produced directly from the DNA (without splicing) taking place as there are no introns in prokaryotic DNA
• Describe how translation takes place

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

  A tRNA molecule carrying an amino acid, with an anticodon that's complementary to the first codon on the mRNA, attaches itself to the mRNA by complementary base pairing. A second tRNA attaches itself to the next codon on the mRNA in the same way.

  The two amino acids attached to the tRNA molecules are joined by a peptide bond. The first tRNA molecule gets reused.
• What is the genetic code?

  The sequence of base triplets in mRNA which code for specific amino acids. Each base triplet is read in sequence, seperate from the triplet before it and after it. The code is also non overlapping
• Name 3 features of the genetic code

  The genetic code is degenerate: 1 amino acid can be coded by different codons/triplets

  The genetic code is universal: The same specific base triplets code for the same amino acids in all living things

  The genetic code is non overlapping: each base in the sequence is only read once