biology 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

• 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 

genes

• '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 

function

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

adaptions 

• 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 
• UNIVERSAL 
• 
  

PROKARYOTIC 

• DNA molecule SMALLER
• forms a CIRCLE 
• NOT associated with PROTEIN MOLECULES 
• DON'T have CHROMOSOMES 
• (no membrane bound nucleus)

EUKARYOTIC 

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

SEMI CONSERVATIVE REPLICATION 

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 

• 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.

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 

• 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).

evidence 

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

mitosis diagram:

cancer: 

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 

• 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 

treatment 

• 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 

INTERSPECIFIC 

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

INTRASPECIFIC 

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

• GENOTYPE + ENVIRO. = PHENOTYPE 

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)