DNA Replication and Protein Synthesis

• Explain why DNA replication is necessary

  In order for cells to function, they need a copy of the instructions to form the necessary proteins (DNA). Therefore in order for cells to replicate, DNA needs to replicate.
• Define the term "semi-conservative replication"

  DNA replication results in one old strand and one new strand present in each daughter DNA molecule
• Draw and annotate a diagram to show the sequence of events in DNA replication

  
• Describe the process of DNA replication in a series of bullet points.

  1. Unzipping and unwinding. Enzyme helicase moves along parent DNA and breaks hydrogen bonds between base pairs. The two strands separate and uncoil.2. Single nucleotides are attached by hydrogen bonds according to base pairing rules to each strand by polymerase 3. Joining the nucleotides. Polymerase catalyses the formation of phosphodiester bonds between each nucleotide to form a new complementary strand alongside the template parent strand.4. Recoiling. Once replication complete, the daughter strand coils to form a double helix.
• State the roles of the enzymes DNA polymerase and DNA helicase in DNA replication

  DNA helicase - breaks the hydrogen bonds between the base pairs in the parent molecule - "unzipping"DNA polymerase - catalyses the condensation reaction between nucleotides to form phosphodiester bonds and make the sugar phosphate backbone of the new strand
• Outline the experimental procedure used by Meselson and Stahl to prove that DNA replicated by semiconservative replication

  E coli. cells grown for many generations in a heavy isotope (N-15) so all cells would have N-15 DNA. Then moved into medium containing N-14. Samples of bacteria removed after each round of division to analyse the DNA by centrifuging which separates according to mass. If it was semi conservative you would get DNA containing one N14 and one N15 strand but if conservative then they would be either two N15s or N14s.
• Describe how, and explain why, DNA replication occurs by continuous replication of one strand and discontinuous replication of the other strand


  DNA polymerase only binds to the 3' end so moves along the template strand in the direcion 3 to 5. This means the formation of the complementary strand along one parent strand is a continuous process (leading strand) and the other is discontinuous (lagging strand). It is built in sections called okazaki fragments which are then joined by DNA ligase.
• Explain the importance of DNA replication conserving genetic information with accuracy

  If there are mistakes in the replication that cause the base sequence to change, it may code for a different amino acid and therefore change the structure of the protein. It may then not function properly and cause issues for the organism.
• Define the term "mutation"


  a change in the genetic material which may affect the phenotype of the organism
• Outline how the sequence of bases in DNA can code for the primary structure of a polypeptide chain

  The sequence of bases codes for a sequence of amino acids. The primary structure of a polypeptide chain is the sequence of amino acids.
• Define the terms "codon" and "gene"

  Codon - a three base sequence of DNA or RNA that codes for an amino acidGene - a section of DNA that contains the complete sequence of bases to code for a protein
• Define the terms "triplet code", "non-overlapping", "degenerate", and "universal" in relation to the genetic code

  Triplet code - the genetic code is a sequence of three nucleic acid bases which form a codon. Each codon codes for an amino acid.Non-overlapping - The code is read from base 1 and the three bases are not present in the next codon - they do not overlap.Degenerate - Several different combinations of bases can code for the same amino acidUniversal - All organisms use the same code
• Explain why the genetic code is a triplet code, the value of it being non-overlapping and the reason for it being degenerate

  It is a triplet code as there are only 4 bases. If each codon was 2 bases there would only be 16 combinations, three bases gives 64 so there is plenty of information to code for 20 amino acids. There are 4 different bases and 4^3 possible combinations of 3 bases but only 20 amino acids so the code is degenerate.
• Describe what is meant by a "start codon" and a "stop codon"

  The start codon signals the start of a sequence that codes for a protein and the stop codons signal the end of a sequence.
• Outline how mutations can alter the structure of a protein

  A change in the base sequence of DNA changes the amino acids it codes for and therefore the primary structure of the protein. As all levels of structure of a protein are dependent on the primary structure, it can seriously affect the function of the protein.
• Define the terms "transcription" and "translation"

  Transcription - the process of copying sections of DNA base sequence to form smaller molecules of mRNA that can be transported out of the nucleus via nuclear pores to the site of protein synthesisTranslation - the process by which the complementary code carried by mRNA is decoded by tRNA into a sequence of amino acids. This occurs at a ribosome.
• State the three types of RNA

  messenger - mRNAtransfer - tRNAribosomal - rRNA
• Compare the structure and function of mRNA and tRNA

  mRNA is a long, unfolded polynucleotide. It is a single strand and uses uracil instead of thymine. It is made at the side of a length of DNA by transcription and then travels to a ribosome where the information is translated into an amino acid sequence to form the specific polypeptide.tRNA is a single strand that forms a cloverleaf structure held in shape by complementary base pairing in parts of the structure. At one end are three unpaired bases known as the anticodon, at the opposite end is the site for the attachment of an amino acid. Each tRNA molecule bonds to a specific amino acid and delivers it to the right place by forming a brief attachment between its anticodon and the corresponding codon of the mRNA so that 'message' is decoded and the polypeptide is formed.
• Draw and annotate a diagram to outline the process of protein synthesis

  
• Draw and annotate a diagram to show the sequence of events in transcription

  
• Describe the process of transcription in a series of bullet points

  DNA unzipsRNA nucleotides line up along template strandJoined by RNA polymeraseWhen stop codon reached, RNA breaks away from DNA which recoils
• Define the terms "sense strand", "antisense strand", and "template strand"


  Sense strand - the strand of DNA which runs 5' to 3' and contains the genetic code for a protein Antisense strand - the strand of DNA that runs 3' to 5' and is complementary to the sense strand. It acts as a template strand during transcription.Template strand - the antisense strand of DNA which acts as a template during transcription so that the complementary strand of mRNA formed carries the same code for a protein as the DNA sense strand
• Describe the structure of a ribosome and explain the role of rRNA

  Ribosomes are made up of two subunits, one large and one small composed of protein and rRNA. rRNA is important in maintaining the structural stability of the protein synthesis sequence and catalyses the reaction.
• Draw and label a diagram of tRNA

  
• Draw and annotate a series of diagrams to show the sequence of events in translation

  
• Describe the process of translation in a series of bullet points

  The mRNA binds to the small subunit of the ribosome at its start codon.A tRNA with complementary anticodon binds to the start codon, carrying an amino acid.Another tRNA binds to the next codon carrying its amino acid. A maximum of two tRNAs can be bound at the same time.Peptide bond formed between the two amino acids, catalysed by peptidyl transferase, a component of rRNA so that the first amino acid is transferred onto the second.The ribosome then moves along the mRNA, releasing the first tRNA so that the second becomes the first and the next codon can be translated.