OCR F215 - cellular control

biology f215 cellular control notes

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Lac Operon

Structural genes: Z coeds for beta-galactosidase, Y codes for lactose permease. These are needed to catalyse the hydrolysis of lactose to glucaose and transport glocose into cells. Lactose indices the production of the enzymes

Operator region: O, switches on/off structural genes

Promoter region: P where RMA polymerase binds to begin transcription of structual genes

Regulator gene: I, not part of the operon


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Lac operon

Lactose absent

  • Regulator gene expressed, regulator protein synthesised
  • Repressor protein binds to operator region and covers promoter region, where DNA polymerase normally attaches
  • RNA polymerase cant bind, structural genes arent transcribed so no enzymes are synthesised

Lactose present

  • Lactose binds to allosteric site of repressor protein, changing its shape so it cant bind onto operator region
  • RNBA polymerase can bind to promoter region and transcibe Z and Y
  • This acs as a molecular switch - allows transcription and translation of Z and Y
  • E.Coli bacteria take up lactose into their cells and convert it to glucose
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Homeobox genes

Control development of the body plan of an organism, including polarity and organ position

Homeobox genes are similar in plants, animals and fungi

Homeobox genes produce polypeptides: some are transcription factors and initiate transcription so regulate expression of other genes

Homeobox genes are arranged into Hox clusters

Retinoic acid activates homeobox genes in vertebrates. Its a morphogen. Hoo much retinioc acid can interfere with expression of the genes, causing birth defects

Drosophila - fruit fly

The development of the fruitfly is genetically mediated by homeobox genes

Maternal effect genes determines embryo's polarity - which is anterior and prosterior

Segmentation genes specify the polary of each segment

Homeotic selector genes specify identify if each segment and direct the development of the segment. These are master genes and control networks of regulatory genes. The 2 gene families, complexes that regulate: thorax and abdomen; and head and thorax elements

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Apoptosis - programmed cell death

  • Occurs in multicellular organisms
  • Enzymes break down cell cytoskeleton
  • Cytoplasm becomes dense with stightly packed organelles
  • Cell surface membrane forms blebs
  • Chromatin condenses and nuclear envelope breaks. DNA breaks into fragments
  • Cell breaks into vesicles, taken up by phagocytosis. Cell debris is disposed of

Controlled by a range of cell signals, some from inside, some from outside cells. Signals include cytokines, hormones, growth factors and nitric oxide (NO induces apoptosis by making inner mitochondrial membrane more permeable to H ions, dissipating proton gradient)

Proteins are released into the cytosol and bind to apoptosis inhibitor proteins, allowing process to take place

There is extensive division and proliferation of cells, followed by apoptosis. Excell cels shrink, fragment and are phagocysed so components are reused and no hydrolitic enzymes are released into surrounding tissue

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Apoptosis is tightly regulated during development. It weeds out innefecteve /harmful T-lymphocyes during the development of the immune system

The rate of cells dying should = the rate of cells produced by mitosos. If rates arent balanced: non enough apoptosis leads to tumours, too much leads to cell degeneraion

Apoptosis was first discovered using electron microscopy, distinguishing between necrosis and apoptosis. Leonard Hayflick showed that normal body cells divide a limited number of times and cancer cells are immortal. Normal diploid strains of of human cells are used worldwide to produce viral vaccines eg polio, rubella, mumps, rabies, hepA

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In sexual reproduction, offspring are genetically different from each other and parents. Parents produce gametes, which fuse together to make a zygote. 

The chromoasome number for each gamete needs to be haploid so after fertilisation, the origional number is restrored

Meiosis 1

Prophase 1

  • Chromatin condenses
  • Chromasomes come together into homologous pairs to form a bivalent. Each member has same genes and loci. There is 1 maternal and 1 paternal chromasome
  • Non-sister chromatids cross over eachother at chaismata (points) and can swap sections of chromatids - crossing over
  • Nucleolus dissapates, nuclear envelope disnintergrates
  • Spinds forms, made of protein microtubules
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Metaphase 1

  • Bivalents line up on the spindle equator, attached to spindles fibres via cetromeer.
  • Chiasmata still present
  • Bivalents are arranged randomly with each member facing opposite poles
  • Chromosomes can indepently segregate in anaphase 1

Anaphase 1

  • Homologous chromasomes are pulled to opposite poles by spindle fibres
  • Centromeeres dont divide
  • Chiasmata seperate. Lengths of chromatid that crossed over remain with their nerly-attached chromatid

Telophase 1

  • Animal cells: 2 new nuclear envelopes form around chromosomes. cell divides by cytokinesis. Brief interphase follows
  • Plant cells: Straight from Anaphase 1 to metaphase 2
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Meiosis 2

Prophase 2

  • Nuclear envelope breaks down again
  • Nucleolus dissapeaes, chromasomes condense, spindles form

Metaphase 2

  • Chromaomes arrange on equator. Attached to spindle fibres at centromeres
  • Chromateds are randomly assorted

Anaphase 2

  • Centromeres dvide, chromatids are pled to opp poles by spindle fibres. Chromatids randomly seggragate

Telophase 2

  • Nuclear envelopes reform around haploid daughter nuclei
  • Animals: 2 cells divide to make 4 haploid cess
  • Plants: A tetrad of 4 haploid cells forms
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Meiosis and Variation

Crossing over - Prophase 1

  • Non-sister chromatids wrap around eachother at chiasmata. Chromatids break at these points. Broken ends rejoin to ends of non-sister chromatids in same bivalent.
  • Similar sections are swapped over. These sections contain same genes but different alleles
  • Produces new combinations of alleles on chromatids 
  • Chiasmata remail in splace during m1

Chromasome re-assortment - Metaphase 1

  • Consequence of random disputation of matenal and paternal chromasomes on the equator. Leads to subsequent segregation in A1
  • Each gamete acquires a different mix of maternal/paternal chromosomes


  • Only 1 ovum is released, only 1 sperm fertalises the ovum to make a zygote
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Chromatid reassortment - Metaphase 2

  • Random distribution on equator of sister chromatids
  • Sister chromatid no longer genetically identical due to crossing over
  • How they align in M2 determines how they separate in A2

Mutation - interphase

  • DNA mutations occur when DNA replicates

Allele - alternative version of a gene

Locus - Specific position on a chromasome occupied by a specific gene

Crossing over - When non-sister chromatids exchange alleles during P1

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Genotype - Genetic make-up of an organism

Phenotype -Outward expression of a particular characteristic

Dominant - Characteristic is always expressed as the phenotype

Recessive - Characteristic is only expressed in phenotype in the absence of a dominant allele

Co-dominant - Two alleles of the same gene that are both expressed in the phenotype of a heterozygote

Linkage - Two or more genes located on the same chromosome. They are inherited together because they don't segregate independently at meiosis.

Sex-linkage - If the gene that codes for a characteristic is on a sex chromosome. Mostly on the X chromasome

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Genetic Diagrams - sex linkage

Genes can be found on only the X chromosome. Females have 2X's so the dominant allele will always be dominant over the recessive allele. Men have 1X so can only have a recessive or dominant, not both, so cant be carriers


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Genetic Diagrams - Codominance

Sickle-cell anaemia

All individuals with the disease have the same mutation

HaHa - normal heamoglobin

HaHs - heterozygous carrier

HsHs - sickle-cell anaemia

In heterozygotes, RBC's are made in bone marrow, half the heamoglobin is normal, half sickled. The normal heamoglobin prevents sickling in RBCs when they are in circulation so they are symptomless carriers

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Genetic Diagrans - Codominance


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Interactions between gene loci

Epistasis - interaction of dfferent gene loci so one gene masks/supresses the expression of another gene locus

The genes may: work antagonistically - masking; or work together complmentary


Homozygous presence of a recessive alle can prevent the expression of aother allele at a second locus. The alleles at the first locus are epistatic to the alleles at the second, which are hypostatic

Recessive epistasis - inheritance of flour colour in Salvia

  • 2 gene loci, A/a and B/b on diff chromasomes involved
  • Pink: AAbb is crosed with white: aaBB. F1 generation were purple: AaBa
  • Breeding F1's gave F2 with purple, pink and white flowers, ratio 9:3:4
  • Homozygous aa is epistatic to B/a. Niether B (purple) or b (pink) can be expressed if there is no dominant A
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Epistasis cont

Dominant epistasis - dominant allele at one gene locus masks expression of the alleles at second gene locus. Fruit colour of squash

  • 2 Gene loci, D/d and E/e involved. D allele resits in white fruit regardless of second loci
  • Homozygous dd's with one E allel give yellow fruits, two e's give green fruit
  • 2 white, double hetero's (DdEe) are crossed F1 show: 12 white (D-E-/D-ee): 3 yellow (ddE-): 1 green (ddee)

Working Complementary - Sweet peas

  • white flowered sweet peas crossed: ccRR x CCrr
  • All F1 plants had purpe flowers
  • F2 had purple and white flowers: 9:7
  • Suggests there has to be at least 1 dominant allele for both gene loci for purple (C-R-)
  • All other genotypes produce white flowers
  • The homozygous recessive condition at either locus mask the expression of dominant allele - cc masks Rr
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Coat colour in mice


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Using Chi-squared

Chi-squared tests the null hypothesis - the starting point in examinging results. Based on the assumption that there is no significant different between observed and expected numbers, any difference is due to chance


We look up the calculated value in the distribution table. Test at 3 degrees of freedom and 5%. If our value is smaller than the critical value we accept Ho - the different is due to chance

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Continuous and Discontinuous variation


  • The quantitve differences between phenotypes
  • Wide range of variation within the population, no distinct catagories, Eg height
  • Traits are controlled by 2+ genes
  • Each gene provides an additive component to the phenotype
  • Different alleles have a small effect on the phenotype
  • A large number of different genes have a combined effect on the phenotype - polygenes
  • The genes are unlinked - on different chromasomes


  • Qualititive differences between phenotypes
  • Clear distinguishable catagories eg blood group
  • Different alleles at a single gene locus have large effects on the phenotype
  • Different gene loci have quite different effects
  • Examples: codominance, patterns of inheritance
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Genotype and environmental contributions to the phenotype

Although a plant may have to genetic potential to produce large ears of grain length 15cm, some may not produce such long ears due to lack of water, light ro nutrients. These enviromnetal factors limit the expression of the gene

In humans, intelligence is determined by genes and environment. Children inherit genetic potential, but potential is only realised by stimulation in a learning environment eg home and school. Also aided by good nutrition for growth and development of morgans, including the brain

Polygenic (continuous) traits are influenced more by the environment and monogenic

  • Variation and Selection
  • Whether the environment/humans are doing the selecting, variation within the pop is necessary
  • When the environment changes, the well adapted individuals survive and reproduce, passing on advantageous offspring. The basis of evolutions and natural selection
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The Hardy-Weinberg Principle

A group of individuals has a larger # of different alleles compared to an induvidual. This gives a pool of genetic diversity, measured using the Hardy-Weinberg equation. Migration, selection, genetic drift and mutation can alter the ammount of genetic variation in a population

We can calculate the allele frequencies in populations for domminant and recessive alleles. IT makes these assumptions:

  • Large population (no sampling error)
  • Mating is random within the population
  • No selective advantage of a genotype
  • No mutation, migration or genetic drift


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The Hardy-Weinberg principle - worked example

Cystic Fibrosis

  • p= freq dominant allele CF.
  • q= freq recessive allele cf.
  • p^2= freq genotype CFCF.
  • q^2= freq genotype cfcf.
  • 2pq is freq genotype CFcf.

when 2 CFcf's mate with eachother, resulting genotypes:                           CFCF CFcf CFcf cfcf: p^2+2pq+q^2

The frequency of alleles (p+q)=1, therefore p^2+2pq+q^2 = 1

  • q^2= 1/2000 = 0.0005.
  • q = sq root 0.0005 = 0.022.
  • p+q=1, so p=1-0.022 = 0.978
  • 2pq = 0.043. This means 4.3/100 people are carriers
  • # carriers ip pop of 2000: 2000 x 4.3/100 = 86
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Environmental factors and natural selection

Factors limiting growth of population: space (plants to grow, animals terratory to feed on), availability of food, light, minerals, water, predation and infection by pathogens. These offer environmental resistance. Can be biotic/abiotic

Population size fluctuates around the mean. Large environmental resistance means pop size will shrink, recucing competition so pop will grow. Intraspecific (within pop) competition for food, shelter, mates increases so pop size falls

Better adapted individuals survive. Have an advantage so pass on advantagous characteristics

Selection pressures can determine which animals survive: Rabbits are camoflaged to escape predetation. Certain coat colours are passed onto offspring - stabalising selection.

If the environment changes. the selection pressure changes - if it snows, white furred rabbits are at advantage. They would survive and pass on white fur alleles. The frequency of alleles in the gene pool would change - directional selection, and evolutionary force

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Genetic Drift

Isolating mechanisms prevent populations from freely interbreeding

Geographical barriers eg river/mountains. Seasonal barrier eg climate changes, Reproductive mechanisms (cant physically mate)

Genetic drift

If a population starts with 2 heterozygous parents, by F2 new allele frequences are drastically different and can eliminate A's or a's in one generation

As the pop size decreases, the degree of fluctuation increases. Fluctuations in allel frequency = genetic drift. It can reduce genetic variation and reduce survuval ability in new environment. Can contribute to extinxion

A natural disaster may cause a population bottleneck (shrink to small size). On an island a pop of 30 have now become 2000. 5% have a rare recessive disease. Studies show the chief was heterozygous for the condition and the only carrier. The allele freq was 0.16. it is now 0.23 (Hardy-Weinberg)

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What is a species?

Biological speies concept - a group of similar organisms that can interbreed to produce fertile offspring

  • Concept is problematic when you want to calssify living organsms that dont reproduce sexually
  • Members of a species may look very different (M/F)

Phylogenic species concept - group of organisms that have similar morphology, physilogy, embryology, bahaviour, and ocupy the same niche

  • Closely related organisms have similar molecular structures for DNA,RNA,proteins. Scientists use systematic molecular analysisto campate base sequneces (haplotypes)
  • Organisms whith haplotypes are a clade - hence cladistic approach. Classification corresponds to phylogenic decent and all taxa (groups) are monophyletic
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What is a species? Phylogenics cont

Cladistics - hierachy classificationof species based on evolutionary ancestry

  • Focuses on evolutionary similarities
  • Great importnace of molecular analysis of DNA and RNA sequencing
  • Uses computer programmes and data from nucleic acid sequencing
  • Makes no distinction between extincy and extant species
  • Doesnt use kingdom, phylum or class - evolutionary tree is complex
  • Corfirms Linnaean classification but has reclassified some organisms


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Natural and Atrificial Selection

Natural selection

Better adapted organisms more likely to survive and reproduce, pass on favourable characteristics

Artificial selection

Human (active) selection of organisms with useful characteristics bred. Humans effect the evelution of the species

Artificial Selection - Modern dairy cow

Cows with high milk yields are repeatedly bred to improve milk yield.Cloning: Cows yield is recordered. Progeny of mulls tested - which produced daughters with high yields. Farmers can store semen for artificial insemination. Elite cows are tgiven hormnes to produce eggs. Eggs fertalised in vitro and embryos implanted into surogates

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Artificial selection - bread wheat

Modern bread wheat has more chromasomes than normal grass so has a larger nucleus. It is a hybrid containing 3 distinct genomes from: wild wheat species, wild emmer wheat, and wild goat gass.

Using Linnaean system of classification, all wheats that can inerbreed and the same species. But genetic classification has been used to disprove this.

Wheats are being improved to have improved characteristics: resisant to fungal infections, high protein, stem stiffness, increased yeild

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Selection Graphs


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