Biology: DNA + cell replication

Cell division

the ability for cells to divide in odrer to gorw and replensih old cells

• Unicellular organism: divison prodcues an enitire other organism (binary fission)

• Multicellular organism": undergo division regularly inorder to grow and repair

Eukaryotic cell division

Creates two daughter cells that are identical to the parnt cell intersm fo egentic info

• done through mitosis and cytokinesis to make somatic cells

• Meiosis then produces reproductive cells

Chromosomes + chromotids

a package of DNA found in the nucelus of a cell that contain genetic information

• two chromatids make up 1 chromosomes but before DNA replication 1 chromosome can be just 1 chromatid

What are the phases of Interphase

• G1 phase (5-6 hours)

• S phase ( half of the cycle)

• G2 phase (4-6 hours)

times aren’t set in stone and can very depening on the cell with some cell nots replicating at all

how is cell division regulated in eukaryotes

using cehckpoints that ensure that the cell is at optimal condition to continue to the next phase of cell division

• G1 checkpoint ( between G1 phase and s phase)

• G2 checkpoint ( end of G2 phase)

• M cehckpoint ( before anaphase and during metaphase

hoe was the existance of checkpoint tested

was tested by fusing two cells together at differnt pahses of cell division (S + G1 phase, M + G1 phase) to make 1 cell with 2 nuclei

1. the G1 nucleus in the first fused cell entered the S phase

2. the G1 nuclues in the second fused cell enetered the M phase

concluded that theer are moleules in teh cytoplasm ofcells responsible fri the passing of checkpoints

How do cehck point signal to cntinue cell division

chackpoints are signalled by rythmic fluctuation od concentration and activity of certain molecle in the cytoplasm

Mature Promoting factors

a molecule present in the cytoplasm of a cell in cell divsion that allows for the passing of the G2 checkpoint thar are made of a combination of Cyclin and cyclin dependednt kinase

How is the G2 checkpoint signalled to be done

1. cyclin will begin to accumulate in the cytoplasm and bind to cyclin dependt kinase to create MPF ( MPF activity increased)

2. MPG phosphorylates proteins requires for mitosis hence promoting it

3. cyclin then degrades after mitosis has been rpmoted while the CDK is used again fro the next cell division

Internal signal example: M phase

the M checkpoint is responsible for ensuring that all of the chromosomes are in there proper position and are attached to the correctly microtiubule for anapahse ( irreverable)

• if not passed then anapahse does not occur

• If it is passed then anaphase occurs

External signal exmaples in animals

Cells are unable to divide without the help of essentaila nurients such as growth factors that are realesed by cells to stimulate the divisison of other cells

• platelet derrived growth factors are secreed by plateklets inorder ot promote the division of fibribalst for repair allowing them to pass their G1 phase

what can stop the cell from dividing

if their are no growt factors or too many cells present then cell will stop dividing and will bind to one another to transmitt and inhibitory signal

How doe cancer cell increase cell division

cancer cells do not require growth factors and are bale to continue dividing for ever as they are able to produce their own graoth factors and eveade apoptosis

• immortal

Hela cell lines

Hela cells from henrietta are cancer cell taken from a woman with cancer over 75 years ago that are still dividing to this day

Types of tumours

tumours are bits of tissue that my form from cancer cells

• beginin tumours arr only found at the original site of mutation

• Maliganat tumors invade surroinding tissue and can sprad all over the body

what are the phases in mitosis

• prophase

• prometaphase

• metaphase

• anaphase

• telophase

Prophase

• the chromatin in the chromosomes begin to condense and bead into discerete duplicate chromosomes

• the mitotic spindle forms from the centrosomes which are made of microtubulues

• the nucleolus disappears

Prometaphase

• Microtubules attach to the kinetichors of the centromere on the chromatids usinf kinetochors microtubules

• nuclear envelope disappears

• cnetrosome begin to float to either side of the cell

Metaphase

the microtubules line up the chromosoems along the ceneter of the cell by shrinking and growing

Anaphase

chromatids are pulled apart to the other side of th cell by microbulues attached to the centrosomes

• the cell elongates with the help of non-kinetochors microtubules so that the cell is able to spilt down the middle

Telophase

the nuclear envelope reappears around the chromosomes using the fragments of the old ones

• chromatin begins to decondense

• nucelouslus reappears

• microtubules as depolarised and removed

Cytokinesis

can occur during telophase or at the end of anaphase and is the physcial separation of the cells

• a microtubules ring of actin forms around th cell creating the cleage furrow whu=ich get more and moe furrowed until the cell are everntually pinched off

• uses motor proventien for the movement of the actin

what is the Mitotic spindle

A network of microtubules made of Tubulin that are highly dynamic allows them to shricnk and grow as necessary

• organised by teh centrosome

• responsibel for the separation of te sister chromatids

Aster

a star like formation of the spindle fibres surronding the centrosomes that face taht memebrane and are used for elongation (non- kinetochores)

Kinetochore Microtubules

Kinetochores Microtubules connect to the kinetochors proteins in th centromere of the chromosome eher the two chromatid connect and pulls them appart

How do spindle fibres undergo anapahse

the fribre s[erate the chromatids using an enzyme called seprase to cleave them in half. the microtubules then shorten usinf motor proteins

• motor proteins will bind to the chromatid and essentaill walk it along the microtunules until it reaches the otehr side of the cell

• as the protein moves the fribre left behin is depolymerased and broken down into monomers

Experiment used to test how microtubules shrick

1. microtubulues in a pigs kidney cell were stained yellow

2. used a laser to remove the dye from the middle section of the spindle so it was only at the ends

3. monitored the length of the marks through cell division

results and conclusions

the spinsdle got short at the end attached to the chromosome allwo us to come to the conclusion that the kinetochores is ,oved down teh fibre

How does cytokinesis occur in a plant cell

instead of using ring of action the plant cells form a cell palte in the middle where the cell intends to split

• the plate surrounded by vesicle to form a membrane

• the cell plate then elongate until it reaches the egde of the cell wear it can fuse with the original cell wall

Binary fusion

the process of cel divison ndergone by prokarytotic bacteria and archea ( sometimes singled celled eukaryotes)

1. the chromosome begins to replicate which initaiates cel division

• begins replicating at 1 origin of replication and moves along the chromosome until it meets up

2. the chromosme will them move to either side of the cell and it begins to elongate

3. the cell will be doubled in size when the protein ring pinches inwards to seprate it

Central dogma

the flow of genetic infromation from DNA to RNA to proteins to phenotyes that was prooved in 1956 by francis crick

• involves DNA replication and transcription and translation

• means that a chnage in DNA can effect teh flow of infromation that may expose didffenr traits or possibel muations

Genetic code

Nucleotide encode of 20 differnt amino acids using a sequnce of 3 nucleotides

• they are read by the ribosomal RNA in set of 3 referred to as a codon

• DNA template strand is transcribed into RNA and RNA is translated into proteins

where doe protein synthesis ouccurs in pro vs eukaryotes

Prokayotes: occurs in the cytoplasm of the cell as they do not have nuclei just regoins

Eukarytoes: Transcription and RNA rocessing occur in teh nucleus and translation occurs in te cytoplasm

Semi-conservative model of replication

found by Watson and hock and shows that ecah Daughter strad in DNAreplcation will have one old strand from the parnt and on newly replicated strand

• in secound replication 1 daughter strsnd will split it a strad of similar structure and one containign 2 newly replicated strand indepednet of the original

Conservative modle of replication

states that one parental DNA strand remians entirely intact as is instead just used as a templete for the next strand

Dispersion cell replication

all repliated DNA contains a mixture of parental and daughter strands

Mathew meselom and Fraklin stahl experiment

used to determine of the semi conservative modle was correct by placing bacteria in a solution of N¹⁵ allowing it to fix to DNA

• then transfered into solution with a lighter isotope of Nitrogen

• the DNA was then sample afetr the first and second replications and centrifuged

Mathew meselom and Fraklin stahl results

• the first replication showed more dense parental strand and reduced less dense daughter strand ( strand produced would all have 1 parental strand so the strands would be more dense)

• the second replicate showed more daughter strsnad and their was more less dense material and less parental strands ( some strand produced would not have the parental strand with the dense isotope so their would be more lighter daughter strands

Origins of replication

The point at which a DNA strand begins to repliucate in a chromosome

• Prokaryotes haave one origina nd eukaroytes have multiple

Origins of replication: prokaryotes

the DNA strand in their circle will pull appart at one sectiona dn New DNA strand will form and bind to ecah old strand creating a bubble

• area where nucleotide are added = replication fork

• eventually meet up in the middle as the final part of the DNA strand dettach allow for tge formation of two new DNA molecules

Origins of replication: eukarytoes

Can have around 5,000 to 60,000 origins of replication as the DNA in eukaroytes in very abdundent and very long so starting from one end woul atke 10000s of years to finish

• the strand dettach at the rplication fork forming bubbles and slowly move in oen dirrection until them meet up with the other origin

Step 1 of DNA replication

First DNA begins to unwind in eukaryotes turning from heterchromatin to euchromatin

• dencondense the unwind from around histones in nucleosomes

• when in DNA form Helicase will begin to break the bonds between the bases on the strands to separation ( found on 5’ to 3’ strand)

• Topoisomerase will them be used to break the back bone of the still wound DNA upstream to relive any tension built up as the elicase unwinding the helix ( also reforms the backbone afterwards

step 2 of DNA replication

Single stranded DNA binding proteins will then bind to the nucelotides at the opening /separation site of teh starnds to prvent them from refermoing bonds

step 3 of DNA replication

RNA primase will then attach to ecah stardn eseperately and form a RNA primer which a later stracture will do to form the new DNA strand

• RNA primer is complementary to the strand itself and allows DNA polymerase to forms strands

• one of leading strand and multiple of lagging strand

step 4 of DNA replication

DNA polymerase will the add on the the 3’ end of the RNA primer to form the new stands based off of the bases on the old strand

• new strand will have the opposite polarity and will replicate towards the replicatiom fork as it opens

• both strands are create at the same time

Leading vs lagging strand

Leading strand is 5’ to 3’ so the new strand will be 3’ to 5’ with the 3’ end facing the replication fork allowing the DNA polymerase to follow it as it opens

The lagging strand is 3’ to 5’ so the new starnd will be 5’ to 3’ means it is facing away from the opening rpilcation Fork and DNA polymerase can only add onto the 3’ primer end. to fix this RNA primase create multiple RNA primars along the strdn fro DNA polymerase to add ont creating fragment of the new strand

Okazaki fragments

a fragment created by DNA polmerase of the lagging strand that don’t dirrectly bind to one another and done so that the polymerase can bind to the 3’ end of the strand

DNA ligase

an enzyme used to bind the back bone of the DNA starnd togther either during repilcation in tersm of okazaki fragment aor in repair in tersm of nucleotide excision repair

Step 5 of DNA replication

the exonuelase of the DNA polymerase proof reads the repl;icaing DNA strand to ensure ni mistakes are made

• if they are they the exonuclease tellks the DNA polymerase to back up and reapir the strand then it will continue with replication

• detects frayed 3’ end or incoorect structures

How is does proof redaig effect DNA

DNA polymerase will tend to make a mistake every 100,000 nucleotide and exonuclease reduces this the 1 misttake every 1 × 10¹⁰ nucleotides

what do the effects of mutation depend on

the effect of a mutation dpenends on

• when in the occurs in cell division ( earlier mean more is effected)

• where is occurs in a gene ( will it plasce in a stop codon, a new amnio acid, no amni acid - may lead to genetic inheritance)

Sickle cell

in sickle cell the one of the nitorgenous base pair is switch from a TA pair to a AT pair leading to the creation of a mutated blood cell

• will insert Valine insated of glutamic acid effecting the function f heamoglobin and reducing oxygen delivery

How can DNA be damaged

• UV rays from the sun can damage DNA if nit properly protected leading to skin camcer

• chemical mutagen found in evereyday food

• extreme pH cahnges

How can UV damage be repaired

using nucleotide excision repair

1. the endonuclease will remove the damaged area of the strand

2. the DNA polymerase then fills in the removed area from 3’ to 5’

3. DNA liagse then fuses the backbone of this newly created strand to reapir it

non-homologoud end joining

done when the Helix is broken in half and the damge is too severe so they are rejoined wrong

Homologous recombination

when two broken strand a paired togther with their non sister chromatid in the homologous pair to undergo crossing over

if damage can not be fixed

if damaged can not be fixed then the cell will undergo apoptosis or cell suicide

• plant cells may also slow cell growth in order to prolong life for reproduction

telomere

a repetivitves sequnce of DNA that does not encode for a proteins but is added to the end of DNA to prevent any genetic information from being lost

• found in linear chromosomes

• added because wgen the RNA primer on the leading strand is removed it cannot be replaced as their is no 3’ end to add to at the end of a DNA molecule ( leaves a small gap at the end that shortens the strand)

Telomere structure

a sequnce of TTAGGG over and over at the end of a strsnd to replace where the RNA primer was

• increases with age as our DNA continues to replicate throughout our lifetime

why do we need telomeres

if we continue replucating the strand will get shorter and shorter until the RNA primer created become part of a genetic sequence and can’t be filled in when remoived

• hence loss of genes

How is telomere made

1. an enzyme called telomerase extends the lacking starnd using its own create RNA that is complementary to the other starnd

2. adds the RNA only to the end of the strand so that it can add on neulsotide twoards the orif=ginal fro the 3’ strand of its RNA

what would happen if we didn’t have telomeres

• genes would evetuallt be lost over time and certain possibly vital proetin can’t be made

• plant cell have junk DNA insated of telomeres that they can get rid of as it does not encode for anything