Topic 4

• Define variation


  Differences that exist between individuals
• In what ways can variation be caused?

  It can be caused by genetic factors, such as alleles from mutations.

  It can also be caused by the environment, such as climate.

  Most variation is caused by a combination of genetic factors and environmental factors
• Why is it impossible to study a whole population?

  Because it is too large and would take too long. Instead, scientists use a sample that is representative of the species' population
• Why should the sample taken be random?

  So you can make sure that the sample isn't biased in any way.
• What could you calculate to look at variation between species?

  The mean (average)
• What is standard deviation?

  It tells you how much values in a single sample vary, by measuring the spread of values about the mean
• What does a large standard deviation mean?


  That the values in the sample vary a lot
• What does a small standard deviation mean?

  That the values in the sample varies little
• What can you use standard deviation to draw on a bar chart of mean values?

  An error bar. You would draw it one standard deviation either side of the mean value.
• How is DNA stored in eukaryotic cells?

  1) The DNA is linear and exists as chromosomes

  2)It is stored in the nucleus, and has to be wound up because it is so long

  3) It is wound around proteins called histones which help to support the DNA

  4)The DNA is coiled very tightly to make a compact chromosome
• How is DNA stored in prokaryotic cells?

  1)Prokaryotic cells carry DNA as chromosomes, but the DNA is shorter and circular

  2)It isn't wrapped around histones, so condenses into the cell by supercoiling
• What organelles in eukaryotic cells have their own DNA apart from the nucleus?

  The mitochondria and chloroplasts have their own DNA. It is similar to prokaryotic DNA as it is shorter, circular and isn't wrapped around histones.
• What is a gene?


  A gene is a sequence of DNA bases that codes for a polypeptide or functional RNA
• What forms the primary structure of proteins?

  The sequence of amino acids.
• What determines the order of amino acids in a polypeptide?

  The order of the bases
• How is each amino acid coded for?

  By a sequence of three bases in a gene called a triplet
• What is functional RNA?

  It is RNA molecules other than mRNA that perform special tasks during protein synthesis.

  
  

  tRNA and rRNA
• What is a cell's genome?


  The complete set of genes in a cell
• What is a cell's proteome?


  The full range of proteins that the cell is able to produce
• If a gene doesn't code for polypeptides, what does it code for?

  Functional RNA such as tRNA and rRNA
• What are introns?


  Sections of a gene that codes for polypeptides, that doesn't actually code for amino acids
• What are exons?


  The sections of a gene that actually code for amino acids
• When are introns removed, and why?

  They are removed during protein synthesis so it doesn't affect the amino acid order

  
• Which type of DNA does not have introns?

  Prokaryotic DNA does not have introns
• What is an allele?

  The same gene, but a different version of it
• What makes two genes alleles?

  The order of bases in each is slightly different. so they code for slightly different versions of the same polypeptide.
• How many chromosomes do humans have?

  23 pairs of chromosomes, 46 in total
• What is a homologous pair?


  Pairs of matching chromosomes. You have two number 1's, two number 2's, two number 3's etc.

  
  

  These are described as homologous pairs
• Describe the features of a homologous pair

  Both chromosomes are the same size and have the same genes, although they could be alleles of eachother.

  
  

  Alleles coding for the same characteristic will be found at the same fixed position (locus) on each chromosome
• What is different about RNA compared to DNA?

  It is a single polynucleotide chain, and contains uracil instead of thymine
• What are the types of RNA?

  Messenger RNA

  Transfer RNA

  Ribosomal RNA
• Describe Messenger RNA


  It is made during transcription and is a single polynucleotide chain.

  
  

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

  
  

  In mRNA, the groups of 3 adjacent bases are called codons.

  
• Describe Transfer RNA


  tRNA is involved in translation, and is a single polynucleotide strand folded into a clover shape.

  
  

  It carries the amino acids that are used to make proteins to the ribosomes

  
  

  Every tRNA molecule has a specific sequence of three bases called and anticodon.

  
  

  It also has an amino acid binding site on the other end.

  
• Describe the first stage of protein synthesis - Transcription

  1) Starts when RNA polymerase attaches to DNA at the beginning of a gene

  2)The hydrogen bonds between the two DNA strands in the gene break, separating and exposing some of the bases

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

  4)RNA polymerase lines up free RNA nucleotides along exposed bases on template strand. The free bases are attracted to the exposed bases and they attach to their complimentary base

  5)Once attached, they are joined together by RNA polymerase, forming an mRNA molecule

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

  7)Hydrogen bonds reform once RNA polymerase has passed and coils back into double helix

  8)When RNA reaches a stop signal, it stops making mRNA and detaches

  9) mRNA moves out of nucleus through nuclear pore and attaches to ribosome.

  
• Describe the second stage of protein synthesis - Translation

  1) The mRNA attaches to a ribosome and tRNA carries amino acids to it

  2) A tRNA molecule with an anticodon that's complimentary to the codon on the mRNA attaches itself

  3)A second tRNA molecule attaches itself to a second mRNA molecule in the same way

  4)The two amino acids attached to the tRNA molecules are joined via a peptide bond. The first tRNA molecule moves away

  5)A third tRNA molecule binds to the third mRNA molecule. Its amino acid binds to the first two and moves away

  6)The process continues producing a polypeptide chain until a stop signal occurs

  7)The polypeptide chain moves away and translation is complete

  
• What is pre-mRNA?


  mRNA strands containing both introns and exons
• What process occurs to remove the introns?

  Splicing occurs when the introns are removed and the exons are joined together. This creates mRNA

  
  

  This takes place in the nucleus before translation occurs
• Why is splicing not necessary in prokaryotic cells?

  Because introns don't occur in the DNA of prokaryotic cells.
• What does it mean that the genetic code in non-overlapping?

  Each triplet is read in sequence, completely separate from the triplet before and after it
• What does it mean that the genetic code is degenerate?

  That there are more possible combinations of triplets (64) than the number of amino acids (20). This means more than one triplet can code for the same amino acid.