topic 4 bio

• describe meiosis

  genetically different daughter cells produced by 2 nuclear divisions, from a single diploid parent
• what are the 3 ways variation occurs in meiosis

  independent segregation

  crossing over

  random fertilisation
• what is independent segregation

  in meiosis 1, homologous pairs line up opposite each other in equator of cell, it is random which side of equator maternal and paternal chromosomes lie
• what is formula for calculating number of combination in meiosis

  2^n
• what is crossing over in meiosis

  in meiosis one, homologous pairs line up opposite each other, and chromatids can become twisted around each other, which puts tension causing pairs to break. broken parts of chromatids recombine with another chromatid resulting in new combination of alleles.
• give 3 differences between mitosis and meiosis

  meiosis: 2 nuclear divisions

  haploid cell (one set)

  introduce genetic variation

  mitosis: 1 nuclear division

  diploid cell (two sets)

  genetically identical
• when identifying meiosis, look out for diploid (2n) parent cell becoming haploid (n) cell
• calculation for possible combinations taking into account random fertilisation

  (2n)^2
• define gene

  short section of DNA that contains code for making polypeptide and function RNA
• define locus

  location of particular gene on chromosome
• define allele

  alternative form of gene #
• define chromosome

  where dna is stored
• what do we call chromosome during interphase

  chromatid
• discuss storage in eukaryotes

  in liner shaped chromosome, DNA is wrapped tightly around histones (protein) so that it is tightly coiled and fits in chromosome. This is called nucleosome
• discuss storage in prokaryotic cells

  shorter, circular, not wrapped around histones but is supercoiled to fit
• describe DNA in mitochondria and chloroplasts

  shorter, circular, not histone bound. This is very similar to the DNA in prokaryotic cells
• give three important features of the genetic code

  degenerate

  universal

  non overlapping
• what is a start codon

  first codon which initiates translation
• what is stop codon

  not complementary to anticodon therefore in translation it causes ribosome to detach so translation stops
• why is a degenerate universal code important

  each amino acid coded for by more than one triplet base so if mutation occurs, could still potentially code for the same amino acid (silent mutation)
• why is a universal genetic code important and give an example

  same triplet base could code for same amino acid in all organisms, advantage for genetic engineering. e.g. insulin from bacteria in humans
• why is non overlapping genetic code important

  each base in gene is only part of one triplet base that codes for one amino acid so each codon is read as a discrete unit. meaning that if mutation occurs, only affects one codon so one amino acid.
• define intron

  sections of DNA that don't code for amino acids- only found in eukaryotes. They are spliced out of mRNA
• define exons

  sections of DNA that do code for amino acid
• define genome 


  organism's complete set of DNA (should never be changing)
• define proteome 


  full range of proteins in one cell- should be constantly changing
• what is the function of RNA

  copy and transfer genetic code from DNA to ribosomes
• describe mRNA and its role

  straight line, messenger RNA. Job is to copy gene from DNA, created in nucleus and leaves the nucleus to carry the copy of genetic code of gene to ribosome (in cytoplasm).
• why does mRNA carry copy of gene rather than DNA

  DNA is too large but mRNA is shorter as it is only a copy of one gene
• describe tRNA and its role

  clover shaped, only found in cytoplasm. its function is to attach amino acid and transfer to ribosome to create polypeptide chain. Specific amino acid attached to specific tRNA. tRNA is complementary to mRNA
• what is rRNA

  combines with protein to make ribosome
• compare monomer of DNA and RNA

  DNA: thymine

  pentose sugar

  deoxyribose

  RNA: uracil

  pentose sugar

  ribose
• compare polymer of DNA and RNA

  DNA: larger

  double stranded

  RNA: shorter

  single stranded
• give brief description of transcription

  one gene copied into mRNA in nucleus
• give brief description of translation

  mRNA attaches to ribosome and tRNA brings to specific amino acid the codon for
• full steps of transcription

  1. DNA helix unwinds exposing bases
  2. one chain acts as a template
  3. DNA helicase unwinds catalyse
  4. DNA helicase breaks the hydrogen bonds between bases
  5. free mRNA complementary align to DNA base
  6. enzyme RNA polymerase bonds together RNA nucleotides to create RNA polymer chain so entire gene is copied.
• what happens during pre-mRNA

  introns spliced out by spliceosomes leaving behind exons (coding region)
• full steps of translation

  1. modified mRNA left nucleus and attaches to ribosomes in cytoplasm
  2. ribosomes attach to start codon
  3. tRNA (with complementary anticodon to start codon) aligns opposite mRNA
  4. ribosomes move along mRNA so complementary tRNA attaches to next codon on mRNA
  5. 2 amino acids are joined by peptide bonds, catalysed by enzyme requiring ATP
  6. continues until ribosomes reach stop codon, so ribosome detaches and translation ends
  7. polypeptide chain created will enter Golgi body for folding and modification