Nucleic Acids & Cell Division

Nucleotide Structure & Nucleic Acids

  • individual nucleotides are made of 3 components combined by condensation reactions
    • phosphate group= consistant structure in all nucleotides
    • pentose sugar
    • organic base containing nitrogen
      • pyrimidine bases= thymine & cytosine
      • purine bases= adenine & guanine
  • DNA is a double stranded polymer of nucleotides or polynucleotide
  • each polynucleotide is made up of millions of nucleotide units
  • in the form of a double helix maintained by hydrogen bonds between bases
  • pentose sugar is always deoxyribose
  • four organic bases in complementary pairs-
    • cytosine & guanine / adenine & thymine
  • DNA  is a very stable molecule and can pass through generations without change
  • very large molecule carrying large amount of genetic info
  • two strands separate easily as hydrogen bonds are weak 
  • base pairs held within deoxyribose-phosphate backbones & is therefore protected
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RNA structure

  • single stranded polynucleotide with pentose sugar ribose
  • RNA has organic bases cytosine & guanine, adenine & uracil
  • mRNA- long single stranded molecule formed into a helix manufactured in nucleus carryig genetic code from DNA to ribosomes in cytoplasm
    • rRNA- found in cytoplasm as a large complex unit of double and single helices e.g ribosomes have rRNA bound to protein. site of translation of the genetic code
      • tRNA- small single strand molecule forming a clover shape with c-c-a sequence at one end for amino acid to attach and anticodon at the other end. transports amino acids to rbosome for protein synthesis


  • sugar is deoxyribose
    • sugar is ribose
  • bases are cytosine & guanine; adenine & thymine
    • bases are cytosine & guanine; adenine & uracil
  • double helix
    • single helix
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Chromosome Structure

  • made from DNA, protein and a small amount of RNA 
  • DNA occurs as a single strand of a double helix running the lenth of the chromosome
  • each DNA molecule has manyy sections of genes
  • before cell division each DNA molecule makes a copy of itself, forming two identical chromatid threads that lie parallel to eachother and are joined at the centromere
  • chromsome numbers vary- in humans it is 46 chromosomes in 23 homologous pairs
  • this is the diploid number
  • half of the diploid number is the haploid number found in gametes - 23 chromosomes
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  • two daughter cells genetically identical to the parent cell
  • dividing cells undergo a cell cycle of 4 stages plus a resting stage between each division


  • longest part of the cell cycle
  • newly formed cell increases in size and produces organelles
  • quantity of DNA is doubled
  • before the next division the chromosomes replicate to form two chromatids joined at the centromere
  • ATP is needed
  • chromosomes are not visible as chromatin is dispersed throughout the nucleus
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Prophase & Metaphase

  • longest stage in mitosis
  • chromosomes condense to become visible as long threads- known as chromatids
  • centrioles divide to reach the poles, synthesise protein microtubles to make spindles
  • nuclear membrane disintergrates and nucleolus disappears
  • chromatid pairs are free in the cytoplasm


  • chromosomes arrange themselves at equator and attach to spindle fibres by centromere
  • contractions of the spindles pull chromatids slightly apart
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Anaphase & Telophase


  • rapid stage
  • centromere splits
  • spindle fibres contractions pull the separated chromatids to the poles centromere first


  • chromosomes have reached the poles, uncoil and lengthen
  • spindle breaks down, nucleoli disappear and nuclear membrane reforms
  • cytokinesis constructs the new cell membrane from outside to in (animals)
  • cell plate laid down from inside to out (plant)
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  • two cells with the same number of chromosomes as parent
  • genetic stability through making genetic identicals to parent
  • growth of an organism
  • repair of tissues/ replacement of dead cells- eg root tip/ skin cells
  • asexual reproduction
  • yeast bacteria insects 
  • flowering plants - bulbs runners tubers
  • large numbers of offspring in a short amount of time
  • no variation
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  • 2 gametes fuse to form a zygote
  • need haploid numbers in these gametes to keep chromosome number stable
  • meiosis 1- chromosome nuber is halved
  • meiosis 2- mitosis of the hapliods for identical daughters
  • result of 4 daughter nuclei with each half the number of chromosomes of the parent
  • prophase 1- homologous chromosomes wrap around eachother and partially repel but remain joined at certain points called chiasmata 
  • chromosomes may break and recombine- this is crossing over and a source of genetic variation
  • during first stage of metaphase chromosomes arrange themselves randomly at equator
  • random distributions produces new genetic combinations
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Mitosis & Meiosis

Mitosis    Meiosis

  • one division 2 daughter cells
    • two divisions four daughter cells
  • number of chromosomes is constant
    • number of chromosomes halved
  • homologous chromosomes not in pairs
    • homologous chromosomes pair up
  • no crossing over
    • chiasmata form and crossing over occurs
  • daughter cells genetically identical
    • daughter cells genetically different
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  • for a species to survive in a constantly changing enviroment and to colonise new environments variation is essential
  • sexual reproduction mixes the genotypes of two parents
  • independant assortment means gametes with different combinations of chromosomes
  • during crossing over chromosome parts are exchanged producing new combinations and separating linked genes
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