biology DNA

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  • Created by: charlie
  • Created on: 26-11-13 20:23

properties of DNA

  • carries info (GENETIC CODE) from essential proteins made - called protein synthesis 
  • DNA REPLICATION - copies itself for growth, repair, reproduction 
  • DeoxyriboNucleic Acid - stable polynucleotide (not denature until approx. 90 d.c)
  • Eukaryotic cells - DNA linear + attached to organising proteins (HISTONES) 
  • Prokaryotic cells (bacteria) - organised in circle loop 

polymer with 3 components (DNA IS A POLYNUCLEOTIDE STRAND)

  • deoxyyribose (sugar) 
  • phosphate 
  • nitrogenous base (adenine, thymine, guanine, cytosine)
  • nucleotides arranged in double helix 
  • two sides are alternating sugar-phosphate groupds  
  • 'rungs' are bases bonded by hydrogen bonds (separates during synthesis + replication) 
  • only one side of DNA is used to make proteins - SENSE STRAND containing triplet sequence of bases coding for A.A - CODONS 
  • other side is to stabilise- NONSENSE STRAND
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bases + genes

  • A = T help by two hydrogen bonds (ADENINE = THYMINE)
  • G   C held by three hydrogen bonds (GUANINE   CYTOSINE)
  • lots of hydrogen bonds make DNA stable 


  • 'section of DNA the codes for the manufacture of a particular polypeptide or protein' 
  • LOCUS/LOCI - position of a gene
  • ALLELES - diff. versions of the same gene arise by mutation
  • INTRONS - sections transcribed but removed from final polypeptide chain 
  • EXONS - exposed section of gene that code for A.A
  • GENE (triplet code or codon) codes for A.A(GTG) ---> A.A sequence codes for polypeptide 
  • 20 diff A.A 
  • multiple repeats - non coding DNA that consists of same base sequence occuring again and again (GTG,GTG,GTG...)
  • KARYOTYPE - charactersation of chromosomes shown in homologous pairs ordered in descending size + relative positions of centromere 
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function + adaptation + triplet code


- 'hereditary material responsible for passing genetic info from cell to cell and generation to generation'


  • VERY STABLE - pass through generations without change 
  • 2 STRANDS JOIND BY H-BONDS - separate easily 
  • LARGE MOLECULES - carry lots of genetic info 
  • PROTECTED FROM OUTSIDE CHEMICALS + FORCES - base pairs are within helical cyclinder of deoxyribose-phosphate backbone 

features of triplet code 

  • most A.A have 3-4
  • DEGENERATIVE CODE - as most have more than 1 
  • NON-OVERLAPPING - each base read once 

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prokaryotic + eukaryotic


  • DNA molecule SMALLER
  • forms a CIRCLE 
  • NOT associated with PROTEIN MOLECULES 
  • (no membrane bound nucleus)


  • DNA molecule LARGER 
  • forms a LINE with HISTONES 
  • IN association with PROTEIN MOLECULES 
  • FORMS structures called CHROMOSOMES 
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mitosis - cell division


USED IN  : growth + repair asexual reproduction 

PRODUCES : genetically idenctical cells 

5 phases 

  • INTERPHASE - DNA replicates (invisible)
  • PROPHASE - chromosomes condense + appear, nuclear membrane dissolves, mitotic spindle (microtubules) form + attaches to kinetochores on each chromosome
  • METAPHASE - moved by microtubules to equator of cell 
  • ANAPHASE - microtubules shorten drawing chromatids of each chromosome to opposite ends + unattached microtubules elongate which stretches cell
  • TELOPHASE - chromatids now called chromosomes reach poles, nuclear membrane reforms as DNA unwinds, 
  • (CYTOKINESIS) - division of cytoplasm between two new nuclei 
  • e.g. animals clevage furrow forms pinching through from each centre until divided into 2 daughter cells e.g. plants is different due to rigid cell wall 
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meiosis - cell division forming gametes

  • PROPHASE 1 (firtst stage)- diploid cells 2 homologous pairs of chromo. (2n)  orginal female parent (MATERNAL homologues) + original male parent (PATERNAL homologues) 
  • PROPHASE 1 (pairing stage) - chromo. pair up, twisted around eachother (BIVALENT), DNA on one chromo. matched exactly to the other 
  • PROPHASE 1 (last stage) - crossing over as homologues break + rejoin at opposite ends, + pull apart slightly, chromatids lie across each other at CHIASMA
  • METAPHASE 1 - nuclear envelope disappeared + spindle formed from CENTRIOLES, attach to centromeres + pull bivalents to equator - INDEPENDENT ASSORTMENT
  • ANAPHASE 1 - spindle fibres shorten, pulling centromeres of a bivalent to opposite poles 
  • TELOPHASE 1 - spindle disappears + nuclear envelope forms around haploid chromosomes
  • INTERPHASE - chromosomes decondense, no DNA replication, pair of centrioles next to nucleus replicate, CYTOKINESIS may/may not occur 
  • PROPHASE 2 - centrioles separate + form spindles , nuclear envelope disappears
  • METAPHASE 2 - chromo. attached to spindle equator by centromeres, centromeres replicate + pull apart slightly as spindle fibres shorten 
  • ANAPHASE 2 - spindle fibres shorten + centromeres pulled to opposite poles 
  • TELOPHASE 2 - spindle disappear + form nuclear envelope around each 4 haploid cells of chromo.
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meiosis - variation

causes of variation from meiosis: 

1. cross over 

  • genes swapped between homologous chromosomes at points called chiasmata 
  • new combination of alleles 

2. independent assortement 

  • arrangements possible with homologous chromosome pairs = 2^(n-1)
  • from maternal and paternal chromosomes

another cause of variation (not meiosis):

3. random fertilisation

  • any female gamete can join with any male gamete 
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semi conservative replication

  • hydrogen bonds holding DNA together broken by enzyme DNA HELICASE 
  • 2 strands separate exposing the bases 
  • bonds between sugar + phosphate groups in polynucleotide strand are strong 
  • enzyme DNA POLYMERASE attaches free nucleotides to exposed bases on template strand
  • new strand built on orginal, therefore 2 DNA molecules are identical 
  • semi-conservative as two molecules have one orginal polynucleotide strand + one new one from supply of nucleotides in the cell (one strand from each molecule is conserved).


  • Meselon + Stahl experiments 
  • centrifuged N15 (heavy) + N14 (light) - shown in tube by density separating out 
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mitosis - Cell cycle

mitosis diagram:


affected by 2 groups of genes:

  • PROTO-ONCOGENES - stimulate cell division + inhibit cell death 
  • TUMOUR SUPPRESSOR GENES - prevent cell division + lead to cell death 
  • mutated proto-oncogenes are ONCOGENES which cannot be controlled by t.s.g so stimulate cell division resulting in tumour 
  • mutated forms of t.s.g cannot control proto-oncogenes resulting in tumour 
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  • common in older somatic (body) cells as accumulate mutations 
  • mutations in gametes passed to next generation - offspring have GENETIC PREDISPOSITION 

enviro. factors (Carcinogens)

  • tobacco SMOKE  + DIET - alcohol, low fibre, high red meat, high animal fat 
  • RADIATION - UV, ionising radiation + 
  • CHEMICALS - asbestos, diesel fumes + MICROORGANISMS - viruses 


  • SURGERY - cutting out but difficult to tell margins + dangerous 
  • RADITHERAPY - xrays or place material in/next to tumour 
  • CHEMOTHERAPY - injection of drugs travel everywhere. targets rapid mitosis in tumour however affects other areas with rapid mitosis (bone marrow sking, gut lining). heals after
  • BENIGN : tumours enclosed + growth in centre (not cancerous) 
  • MALIGNANT : growth at edges invade surrounding areas, difficult to tell boudaries. METASTASIS where cells break + move elsewhere resulting in 2nd tumour 
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types of variation


  • differences in DNA 
  • structure + sequence of GENES in organsim + the way theyre EXPRESSED 


  • GENOTYPE - genetic makeup + collection of alleles 
  • PHENOTYPE - collection of observable features 

types of intraspecific variation 

  • DISCONTINUOUS - one category or another. controlled by single alleles 
  • CONTINUOUS - range of values. most fall into mid range (NORMAL DISTRIBUTION).  contolled by several genes (POLYGENIC). more affected by enviro. 
  • STANDARD DEVIATION = difference from mean value 
  • IDENTICAL TWINS (monozygotic) NON IDENTICAL TWINS (dizygotic)
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