as biology unit 2 revision cards

8/3/2019 AS Biology Unit 2 Revision Cards

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Eukaryoticcells

Containdiscretemembrane-boundorganellessuchasnuclei,mitochondriaandchloroplasts.

Eukaryoticcellsarelargerthanprokaryoticcells.Notallprokaryoticcellshaveacellwall.

Ageneralisedanimalcellconsistsof:

Nucleus-containschromosomesandanucleolus,theDNAinchromosomescontains

genesthatcontrolthesynthesisofproteins.

Nucleolus-Adensebodywithinthenucleuswhereribosomesaremade

Roughendoplasmicreticulum(rER)-asystemofinterconnectedmembrane-bound

flattenedsacs.Ribosomesareattachedtotheoutersurface.ProteinsmadebytheseribosomesarethentransportedthroughtheERtootherpartsofthecell.

Ribosomes-madeofRNAandprotein,theyarefoundinthecytoplasmorattachedto

theendoplasmicreticulum.Theyarethesiteofproteinsynthesis.

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Prokaryotic cells

Prokaryotic cells do not have nuclei or other membrane-bound cell organelles. Examples of a

prokaryotic cell are bacteria and cyanobacteria.

infolding of cell surface membrane (site of respiration)

plasmid (small circle of DNA)

capsule (slimy layer on surface for protection and to prevent dehydration)

pili (thin, protein tubes allow bacteria to adhere to surfaces) flagellum (hollow cylindrical thread-like structure that rotates to move the cell)

robosome

circular DNA

cell surface membrane

cell wall (contains peptidoglycan, a type of polysaccharide and peptide combined)

cytoplasm

Most prokaryotics are very small. Their DNA is not associated with any proteins and lies free in

the cytoplasm. A cell wall is always present in a prokaryotic cell.


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Releaseofproteins

Innucleus:

TranscriptionofDNAtomRNA

mRNAleavesthenucleusthroughaporeinthenuclearenvelope

Incytoplasm:

ProteinthatismadeonribosomesenterstheroughER

theproteinmovesthroughtheERassumingthree-dimensionalshapeenroute

vesiclespinchedofftheroughERcontaintheprotein

vesiclesfromroughERfusetoformtheflattenedsacsoftheGolgiapparatus

proteinsaremodifiedwithintheGolgiapparatus

vesiclesarepinchedofftheGolgiapparatuscontainthemodifiedprotein

vesiclefuseswiththecellsurfacemembranereleasingprotein,suchasextracellular

enzymes.

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Eukaryotic cells continued

A generalised animal cell consists of:

Golgi apparatus - sacks of flattened membrane-bouns sacs formed by fusion of vesicles

from the ER. Modifies proteins and packages them in vesicles for transport.

Lysosome - spherical sacs containing digestive enzymes and bound by a single

membrane. Involved in the breakdown of unwanted structures within the cell, anddestruction of whole cells when old cells are to be replaced or during development.

Centrioles - Every animal cell has one pair of centrioles. They are involved in the formation

of the spindle during nuclear division and in transport within the cell cytoplasm.

Cell surface membrane - phospholipid bilayer containing proteins and other molecules

forming a partially permeable barrier.

Smooth endoplasmic reticulum (sER) - like rough ER, but does not have any attached

ribosomes. Smooth ER makes lipids and steroids.


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Whenthespermmeetstheovum

spermreachovum

chemicalsreleasedfromcellssurroundingovumtriggeringtheacrosomereaction

theacrosomeswellsfusingwiththespermcellsurfacemembrane

digestiveenzymesintheacrosomearereleased

theenzymesdigestthroughthefolliclecells

andthejelly-likelayersurroundingtheovum

thespermfuseswiththeovummembrane

thespermnucleusenterstheovum

enzymesreleasedfromthelysosomesintheovumthickenthejelly-likelayer,preventing

entryfromothersperm

nucleioftheovumandspermfuse

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Gametes

Gametes are highly specialised, with structures and functions that are different from other body

cells. The sperm and ovum (egg) cells are the gametes (sex cells), the gametes are adapted for

their roles in sexual reproduction.

Ovum:

large cell that is incapable of independent movement it is wafted along by ciliated cells lining the tubes and by muscular contractions of the tubes.

the cytoplasm of the ovum contains protein and lipid food reserves for a developing embryo

surrounding the cell is a jelly-like coating called the zona pellucida

Sperm:

much smaller and can move by itself

to enable it to swim the sperm has a long tail powered by energy released by the

mitochondria

the sperm consists of the acrosome, nucleus and the mitochondrion


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Howdogametesform?

Mitosisproducesnewbodycellsasanorganismgrowsanddevelops.Thisretainsthefull

numberofchromosomes,calledthediploidnumber(46chromosomes).

Meiosisproducesgameteswithonlyhalfthenumberofchromosomes,calledthehaploid

number(23chromosomes).

Gameteproductionbymeiosis:

startswithhomologouspairofchromosomes

chromosomesreplicatebeforedivision.afterreplicationeachchromosomeismadeupoftwostrands

ofgeneticmaterial,twochromatidshomologouspairofreplicatedchromosomesmade(chromatids)

homologouschromosomespairupandthenseparate

chromatidsseparateandgametesareformed,eachwithhalftheoriginalnumberofchromosomes.

Meiosishastwoimportantrolesinbiology.Itresultsinhaploidcells,whicharenecessaryto

maintainthediploidnumberafterfertilisation.Secondly,ithelpscreategeneticvariationamong

offspring.

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Chromosomes

Human cells contain 46 chromosomes made up of 22 homologous pairs and one pair of

sex chromosomes.

Fruit fly cells contain 8 chromosomes made up of 3 homologous pairs and one pair of sex

chromosomes.

White campion plant cells have 24 chromosomes made up of 11 homologous pairs andno sex chromosomes (as separate male and female individuals are not produced)

Gametes have half the number of chromosomes found in normal cells (one chromosome from

each homologous pair).

The sperm and the ovum each contain 23 chromosomes, made up of one of each homologous

pair and one sex chromosome. So when the gametes fuse the full number of 46 chromosome

sis restored.

Haploid gametes come together to form a diploid zygote.


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Howdoesmeiosisresultingeneticvariation?

continued

Crossingover:

Duringmeiosis1,homologouschromosomespair.Atpointswheretheymakecontact,called

chiasmata,thechromatidsbreakandrejoinexchangingsectionsofDNA.Thenon-sister

chromatidsexchangecorrespondingsectionsofDNA.

Maternalandpaternalchromosome

chiasma,siteofcrossingover

theycrossover

andthehomologouschromosomesseparate

thechromatidsthenseparate

crossingoverproduceschromosomesthatcontainnewcombinationsofallelesfromboth

parents

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How does meiosis result in genetic variation?

The suffling of existing genetic material into new combinations during meiosis is important in

creating genetic variation.

This shuffling includes:

independent assortment

crossing over

Independent assortment:

during meiosis only one chromosome from each pair ends up in each gamete

the independent assortment of the chromosome pairs as they line up during meiosis 1 is

a source of genetic variation

this process is random; either chromosome from each pair could could be in any gamete

an organism with 6 chromosomes, that is the three homologous pairs XX, YY and ZZ could

form eight combinations in its gametes,


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Fertilisationinplants

Inplants:Infloweringplants,nucleifromthegametesalsohavetocombineintheprocesso

fertilisation.

fertilisationtakesplaceintheembryosacwithintheovule

thepollengraingerminatesonthestyle

apollentubegrowsdownthroughthestyletowardstheovary,withitsgrowthcontrolled

bythetubenucleus.

thepollengraincontainstwonuclei(thetubenucleusandthegenerativenucleus

ongerminationofthepollen,thegenerativenucleusdividestoformtwohaploidgamete

nucleiwhichmovedownthepollentube.

thetubegrowsthroughamicroscopicporeintotheembryosacandthetwomalegametenucleienterthesac.

onefuseswiththeeggcellandformsadiploidzygote

thesecondfuseswithtwonucleiintheembryosaccalledpolarnucleitoformatriploidcell.

Thisdiploidzygotedividestoformtheembryo.Thetriploidcelldividestoformtheseed'sstorage

tissue,endosperm.

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Fertlisation in mammals

In mammals:

To produce a new individual, the nuclei from the gametes have to combine in the process of

fertilisation.

In mammals the nucleus from one sperm enters the ovum

and the genetic material of the ovum and sperm fuse forming a ferlilised ovum called a zygote

This cell now contains genetic material from both parents.


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Celldivision

Prophase:

Thechromosomescondensebecomingshorterandthicker

witheachchromosomevisibleastwostrandscalledchromatids.

Thetwostrandsareidenticalcopiesofoneanother,producedbyreplication.

Theyaretwochromosomesjoinedatonerejoincalledthecentromere.

microtubulesfromthecytoplasmformthree-dimensionalstructurecalledthespindle.

thecentriolesmovearoundthenuclearenvelopeandpositionthemselvesatappropriate

sidesofthecells.

theseformthetwopolesofthespindle,andareinvolvedintheorganisationofthespindle

fibresthespindlefibresformbetweenthepoles.(thewidestpartofthespindleiscalledthe

equator.

Metaphase:

thebreakdownofthenuclearenvelopesignalsthestartofmetaphase

thechromosomes'centromereattachtospindlefibresattheequator

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The cell cycle


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Whyismitosissoimportant?

Mitosisensuresgeneticconsistencyby:

makingdaughtercellsgeneticallyidenticaltoeachother

andidenticaltotheparentcell

eachdaughtercellcontainingexactlythesamenumberandtypeofchromosomesastheir

parentcell

thisisachievedby:

DNAreplicationpriortonucleardivision

thearrangementofthechromosomesonthespindleandtheseparationo

chromatidstothepoles

Thisgeneticconsistencyisimportantingrowthandrepair,andalsoinasexualreproduction.

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Cell division continued

Anaphase:

the spindle fibres shorten

pulling the two halves of each centromere in opposite defections

one chromatid of each chromasome is pulled to each of the poles

anaphase ends when the separated chromatids reach the poles and the spindle breaks

down

Telophase:

The chromosomes unravels

nuclear envelope reforms

the two sets of genetic information become enclosed in the separate nuclei.

Interphase:

new cell organelles are synthesised and DNA replication occurs by the end of interphase the cell contains enough cell contents to produce two new cells.


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Earlyembryonicdevelopment

Cellsintheearlyembryo:

Afterashumanzygotehasundergonethreecompletecellcycles:

itconsistsofeightidenticalcells

cellsaretotipotent(aseachcellcandevelopintoahumanbeing)

fivedaysafterconceptionahollowballofcellscalledtheblastocysthasformed

theouterblastocystcelllayergoesontoformtheplacenta

theinnercellmassofaround50cellsgoesontoformthetissuesofthedevelopingembryo

these50cellsareknownaspluripotentembryonicstemcells

eachofthese50cellscanpotentiallygiverisetomostcelltypes

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Growth, repair and asexual reproduction

Growth and repair:

Mitosis occurs in the growth of any organism

it insures that a multicellular organism has genetic consistency (all the cells in the body

have the same genetic information)

some organisms can regenerate lost parts of their body using mitosis

mitosis also allows old and damaged cells to be replaced with identical new copies

Asexual reproduction:

many organisms grow copies of themselves by mitosis producing offspring that are

genetically identical to each other, and to their parent.

asexual reproduction occurs when bacteria undergo binary fission; the bacterial cell grows

and then divides into new cells. An outgrowth from the parent detaches to form a new

individual.

it takes place in plants too (vegetative reproduction) E.g. bulbs and tubers. some organisms such as mosses, liverworts and all plants that reproduce both asexually

and sexually at one time another.


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Earlyembryonicdevelopment

Potentialuseofhumanstemcellsinmedicine:

Theycouldproduceuniversalhumandonorcellswhichwouldprovidenewcells,tissues,

ororgansfortreatmentorrepairbytransplantation.

pluripotentstemcellsformedicineandresearchcanbetakenfrom'spareembryos'

oneproblemwiththisapproachisthatthetissuemightberejectedbytheimmune

system

therearealsomanyethicalconcernswiththisapproach

therapeuticcloning

adiploidcellisremovedfromthepatient(e.g.fromthebaseofahair)

thiscelloritsnucleuswouldthenbefusedwithanovumfromwhichthehaploid

nucleushadbeenremoved

theresultwouldbeadiploidcelllikeazygote

thisprocessisknownassomaticcellnucleartransfer(asomaticcellisanydiploid

bodycell)

tissuetyping

ithashelpedinthepasttofindasuitabledonorforabloodtransfusion

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Early embryonic development

Cells become more differentiated:

as the embryo develops into a multicellular body the cells from which it is made become

increasingly differentiated. These cells are known as multipotent stem cells, e.g. neural stem

cells can develop into cells found in the nervous system. Differentiation is irreversible in animal

cells.

In a plant:

Most plant cells remain totipotent throughout the life of the plant.

Totipotency of plant cells allows plants to be reproduced using plant tissue culture. Small pieces of

plant (explants), are sterilised and then placed on a solid agar medium with nutrients and growth

regulators.

The cells then divide to form a mass of undifferentiated cells called a callus. By altering the growth

regulators in the medium they can be grown into a full plant.


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Howdevelopmentiscontrolled

Thenucleushasaroleincontrollingthedevelopmentoftheindividualcellandthewhole

multicellularorganism'sphenotype.Thiswasfirstshowninclassicexperimentsusinggiantalgalcells.

TheAcetabulariamediterraneaandtheAcetabulariacrenulatahave:

ahat

astalk

andarhizoid(bottom)containingthenucleus

Ifthehatsareremovedandthestalksswapped,theplantdevelopshatswithfeaturesofboth

species.(intermediatehats)

Iftheintermediatehatsarethenremoved,newonesgrowthatcorrespondtothenucleusinthe

rhizoid.

Thisshowstheimportanceofthenucleusandchemicalmessengersinthedevelopmentofthecell.

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Ethical conserns about the use of stem cells

People agree that there are no ethical objections to using multipotent stem cells taken from adults.

But the problem is that scientists believe that these stem cells are less valuable to their research

than pluripotent stem cells which can only be found in human embryos.

Arguments:

some people believe that an embryo is a human being from the moment it is created so

it should have full rights as a human being

some people argue that an embryo requires and deserves no particular moral attention

whatsoever

others accept the special status of an embryo as a potential human being, but argue that

the respect should increase as it develops. They believe that the importance the embryos

have in stem cell research outweighs the embryos importance.


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Differentgenesareexpressed

Astheembryodevelops,cellsdifferentiate:theybecomespecialisedforonefunctionoragroup

offunctions.Structureandfunctionofeachcelltypeisdependentontheproteinsitsynthesises.

Somepeopledemonstratedthatdifferentgenesareexpressedindifferentcells.Theyextracted

mRNAfromdifferentiatedandundifferentiatedfrogcells.

ThemRNAisextractedfromundifferentiatedcellsinearlyfrogblastula

themRNAindifferentiatedcellsinlaterdevelopment(gastrula)isextracted

complimentaryDNA(cDNA)ismadeusingreversetranscriptase

mRNAisthendigested

thecDNAandthemRNAarecombined

anymRNAthatisalsoproducedinthedifferentiatedcellswillcombinewithcDNAtoform

doublestrands

freecDNAisfrommRNAproducedinthedifferentiatedcells

CellsbecomespecialisedbecauseonlysomegenesareswitchedonandproduceactivemRNA

whichistranslatedintoproteinswithinthecell.

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How 'Dolly' was made

sheep 1: mammary cell donor sheep. mammary cells are grown in culture.

sheep 2: egg cell donor sheep. the nucleus from an egg cell found in the ovary is removed.

cells are fused together

the nucleus from the mammary cell inside egg cell minus its nucleus

it is grown in a culture

an early embryo forms

it is implanted into the uterus of a third sheep

the embryo develops

and a lamb is born that is chromosomally identical to mammary cell donor


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Whatswitchesanindividualgeneonoroffin

eukaryotes?

Genesinuncoiled,accessibleregionsoftheeukaryoteDNAcanbetranscribedintomessenger

RNA.

TheenzymeRNApolymerasebindstoasectionoftheDNAadjacenttothegenetobetranscribed.

Thissectionisknownasthepromoterregion.

OnlywhentheenzymeisattachedtotheDNAwilltranscriptionproceed.

Thegeneremainsswitchedoffuntiltheenzymeattachestothepromoterregionsuccessfully.The

attachmentofaregulatorproteinisusuallyalsorequiredtostarttranscription.

TranscriptionofagenecanbepreventedbyproteinrepressormoleculesattachingtotheDNAo

thepromoterregion,blockingtheattachmentsite.

Proteinrepressormoleculescanattachtotheregulatorproteinsthemselvespreventingthemfrom

attaching.

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Switching on # - galactosidase synthesis

Some French geneticists studied the control of genes in the prokaryote Escherichia coli.

These bacteria only produce the enzyme #-galactosidase to break down the carbohydrate lactose

when it is present in the surrounding medium.This enzyme converts the disaccharide lactose to

the monosaccharides glucose and galactose.

If lactose is not present in the environment:

A lactose repressor molecule binds to the DNA and prevents the transcription of the #-

galactosidase gene.

If lactose is present in the environment:

The repressor molecule is prevented from binding to the DNA, and the #-galactosidase gene is

expressed. mRNA coding for #-galactosidase is transcribed and traslation of this mRNA produces

the enzyme.


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Somekeywords

Cell:

Inmulticellularorganismscellsarespecialisedforaparticularfunction.Forexample,musclecells

andepithelialcells.

Tissue:

Agroupofspecialisedcellsworkingtogethertocarryoutonefunction.Forexample,musclecells

combiningtoformmuscletissue,andepithelialcellsformingepithelialtissue.

Organ:

Agroupoftissuesworkingtogethertocarryoutonefunction.Forexample,muscle,nerveand

epitheliumworktogetherintheheart.

Organsystems:

Agroupoforgansworkingtogethertocarryoutaparticularfunction.Forexample,thecirculatory

system.

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When gene expression goes grong

Growing bones in the wrong places: FOP (fibrodysplasia ossificans progressiva)

FOP is characterised by the growth of bones in odd places, such as within muscles and connective

tissue.

What causes it?:

FOP is an inherited condition caused by a gene mutation. In FOP one of the genes that are used to

produce proteins that make bone cells is not switched off in white blood cells. So when the tissue

is damaged white blood cells move to the site of damage and produce the protein that makes

bone cells.

How are cells organised into tissues?:

Specialised cells can group themselves into a cluster working together as a tissue. Cells have

specific recognition proteins, also known as adhesion molecules, on their surface membranes.

Adhesion molecules help to recognise other cells like themselves and stick to them.


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Genesandenvironment

Thecharacteristicsofanorganism(e.g.height)areknownasitsphenotype.Differencesin

phenotypebetweenthemembersofapopulationarecausedbydifferencesin:

geneticmake-uporgenotype

theenvironmentinwhichanindividualdevelops

Somecharacteristicsarecontrolledbytheorganism'sgenotype,withtheenvironmenthavinglittle

ornoeffect(e.g.bloodgroup).Suchcharacteristicsarecontrolledbygenesatasinglelocus

andshowdiscontinuousvariation.Theyhavephenotypesthatfallintodiscretegroupswithno

overlap.

Characteristicsthatareaffectedbybothgenotypeandenvironmentoftenshowcontinuous

variation(e.g.humanheight).Ifagraphisdrawnshowingthefrequencydistributionofthedifferent

heightcategoriesitwillbebell-shaped.

Characteristicsthatshowcontinuousvariationarecontrolledbygenesatmanyloci,known

aspolygenicinheritance,andalsobytheenvironment,eitherdirectlyorbyinfluencinggene

expression.

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Gene expression and development

The precise sequence of transcription and translation of genes determines the sequence of

changes during development.

Master genes:

Master genes control the development of each segment.The master genes produce mRNA which

is translated into signal proteins. These proteins switch on the genes responsible for producingthe proteins needed for specialisation of cells in each segment.

The ABC of flowering plants:

When a plant starts to flower, cells in a meristem become specialised to form the organs that make

up the flower.Most flowers contain the organs: sepals, petals, male stamens and female carpels.

These are arranges in concentric whorls.

The expression of genes in cells across the meristem determines which structures will form. When

only gene A is expressed sepals form, when only gene C is expressed carpels form. Petals formwhen A and B are expressed, and stamens when B and C are expressed.


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Geneandenvironmentinteractions

Therearemanyexampleofgenesandenvironmentinteractingtogethertoproduceanorganism's

phenotype.Someexamplesare:

height

haircolour

MAOA

causesofcancer

Height:

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