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The Cell Cycle
Karyotype - a display of all of the chromosomes of a cell, can only occur when the chromosomes become very
condensed and coiled
How the chromosomes become condensed and coiled:
1
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Each nucleosome consists of 8 histone proteins that DNA wraps 2
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Super coiling takes place where the nucleosome folds up and wraps to form a more visible condensed
structure with loops
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The fibers/loops are compressed and folded to form a fiber that undergoes tight coiling which results in the
chromatid of a chromosome
STAGES
Movement through the phases is done by cyclin proteins that attach to cyclin-dependent kinases (CKSs)
Interphase - processes when the cell isn’t divided
1
...
Interphase G0 - cell division stops temporarily or permanently, normally occurs where materials are not
available or environmental conditions are not ideal
3
...
Interphase Gap 2 (G2) - a checkpoint where the cell checks for and repairs damage to its DNA before
proceeding to mitosis
5
...
Cytokinesis - the cytoplasm is divided between the 2 daughter cells - each new cell enters G1 and resumes its
normal functions
Phosphorylation - the process of adding a phosphate group to the R of amino acids which promotes transcription in
factor S and causes chromatin to become denser

MITOSIS – forms somatic cells
Mitosis - asexual reproduction and is a form of cell division that produces two daughter cells with the same genetic
component as the parent cell
Prophase
– The chromosomes coil up - each chromosome consists of two daughter chromatids attached at the
centromere – super coiling takes place
– The chromosomes can take up a stain and become visible
– The nucleolus/ nuclear envelope breaks down and the centrioles begin to pull apart, forming spindles
– In plants there is no centriole – spindles are from cytoplasm
Prometaphase
– Chromosomes continue to condense, and kinetochores appear at the centromeres
– The mitotic spindle microtubules attach to the kinetochores
Metaphase
– Nuclear membrane is broken down completely and centrioles have moved to opposite poles
– The chromosomes line up on the metaphase plate (center of cell) – spindle fibers arrange them
Anaphase
– The chromatids on each chromosome are pulled towards opposite poles of the cell
– Certain spindle fibers begin to elongate the cell - ATP produced in cell respiration is required
Telophase
– Chromosomes at opposite poles of the cell begin to decondense/lengthen - harder to see – 2 nuclei
– The mitotic spindle fibers break down and a nuclear envelope forms and surrounds each set of
chromosomes - nucleoli and centrioles are also reformed
Cytokinesis – not part of mitosis
– Final phase where the cytoplasm divides and two genetically identical daughter cells are formed
– In animal cells : a ring of contractile fibers tightens around the center of the cell forming a cleavage furrow,
the fibers continue to contract until the cytoplasm is separated
– In plant cells : a cellulose cell wall builds up from the inside of the cell outwards - a cell plate separates the
daughter cells (end plate)

Mitotic index - measurement of how actively cells are performing mitosis, useful in cancer cell detection - cancer
cells will divide more, so have a higher index --> cells in mitosis / total number of cells

Meiosis
Gametes - haploid sex cells that contain one set of chromosomes compared to a diploid somatic (body) cell
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PROCESS
Interphase (S phase) - this is when the DNA chromosomes are replicated to produce two identical copies (sister
chromatids) that are held together at the centromere by cohesion proteins
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MEIOSIS I
Prophase I
– The homologous chromosomes are attached at their tips to the nuclear envelope by proteins
...
The tight pairing of the chromosomes is called synapsis
...
This is where there is an exchange
of DNA between chromatids on homologous chromosomes
– Chiasmata (twist around each other) forms between non-sister chromatids which allows the exchange of
chromosomal segments between them
...

– Genetic variation – crossed over chromosomes have the same genes, however the alleles are shuffled
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Each
homologous pair attaches to the opposite poles of the cell
...
The tetrads are arranged randomly, and the arrangement is always
different, but each chromosome lies next to its homologous partner -Independent assortment
– When the chromosomes are divided in half, the separation is random; each haploid cell contains both
paternal and maternal genes
...

Anaphase I
– The microtubules pull the linked chromosomes apart, however the sister chromatids are still tightly bound
together at the centromere
...
A
nuclear envelope forms around each chromatid
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Resulting in two haploid
cells, each containing one duplicated copy of each homologous chromosome pair
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New
spindles are formed
Prometaphase II
– The nuclear envelope is completely broken down and spindles are fully formed
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Telophase II and Cytokinesis
– The chromosomes arrive at opposite poles and decondense
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– Cytokinesis separates the two cells into four unique haploid cells by forming a cleavage furrow
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The condensed state reduces the
amount of energy needed to transport it
– Microtubules - produce the 'whip like' movements of the tail that help it swim towards the ovum
– Flagellum - propels the sperm by its movement in a liquid environment
Female Gametes in mammals - ova
The process that produces female sex cells is oogenesis
...

Oocytes form the basis of the gamete that undergoes cell division by meiosis I during puberty
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Second stage(gametophyte generation) - gametogenesis: haploid, produces haploid cells by mitosis
Pollen(Male) - microgametes
microsporogenesis
1
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These diploid cells form four microspores that are haploid (gametophyte generation) which then undergo
gametogenesis
microgametogenesis
3
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The cells don’t split, two nuclei occupy one cell;
pollen tube nucleus and generative nucleus
4
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These sperm nuclei will fuse with an ova cell during fertilization
Ova(Female) - megagametes
megasporogenesis and megagametogenesis
1
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Megasporogenesis - 3 of the 4 megaspores degenerate and only one megaspore is left in each ovule
3
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The three that migrate to the top are called antipodals, their function is to produce lipids which are then
used for energy during later development
5
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The polar nuclei fuse to form a diploid cell which can then be a seed
STEM CELLS
– After fertilization the two haploid gametes have fused to form a diploid zygote - this zygote can differentiate
into any type of cell
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This is a special kind of mitosis where
interphase stages do not occur as the cells divide
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How they become specialized?
– Stem cells all contain the same genes - not all expressed as not all active - under the right conditions, some
genes are activated and other deactivated
– mRNA is only formed/transcribed from the active genes, and then translated to form a polypeptide chain
that folds to form a protein
– The proteins formed modify the cells - determine cell structure and control cell processes e
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more genes
activated; more proteins
– These changes to the cells cause it to become specialized - differentiated
...
g
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By the blastocyst stage - embryo implanted in
its mothers uterus, the inner cells are pluripotent
...
g
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They differentiate
when needed to form any cell in that tissue/organ
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Removing the nucleus from one of the normal body cells and transfer it to a human ovum that has its
original nucleus removed
2
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The new cell divided and produces a collection of identical cells with the same genetic information as the
patient
4
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5
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This is still under development and the stem cells can only be used by the patient who's nucleus was used otherwise it would be rejected

Structural gene - transcribed into mRNA and codes for useful proteins such as enzymes
Regulatory genes - aren't transcribed into mRNA and codes for an activator or repressor
Phenotype is decided by the interaction between the genotype and the environment
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g
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coli is a bacteria that can break down lactose into galactose and glucose using the enzyme betagalactosidase(lactase)
The genes necessary to transcribe lactase are only expressed when lactose is present
When Lactose isn't available/not present:
– The regulatory gene produces the active repressor (lac repressor) which acts as a transcription factor that
binds to the operator region, which prevents RNA polymerase from binding to the promoter region
– So this means that transcription is blocked and no mRNA can be transcribed using genes for lactase
When Lactose is available/present:
– Lactose diffuses into the E
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Since it
can no longer bind, RNA polymerase can now bind to the promoter region and begin transcribing the mRNA
for lactase
– When the mRNA strand is formed and meets a ribosomes it gets translated to form beta-galactosidase
(lactase) which breaks down lactose molecules that enter the E
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Studying Variation in Humans
Studying variation due to environmental factors needs to be ideally done on genetically identical organisms
Twin Studies
– Different groups have measurements taken to see the difference in twins - the larger the difference, the
larger effect that factor has on the genotype
– For example their could be a group of identical twins raised apart, another group of identical twins raised
together, non-identical twins and siblings(not twins)
– Once the difference in measurements for factors such as height, weight, IQ are calculated - the
environmental factors that are different (responsible for the difference) are identified
Epidemiology - the study of distribution and determinants of health-related states or events in populations
Discontinuous variation - studies categories of characteristics (whether they have it or don’t)
...
g
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It is normally the
combined effect of many genes and is often affected by environmental factors
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g
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g
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Referred
to as pre-mRNA
– RNA splicing takes place by removing the intron sections and sometimes even exon sections
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This helps recognition when translation occurs using tRNA
Epigenetics - the study of heritable changes in gene expression that do not involve changes to the actual DNA
sequence
DNA Methylation
– DNA methylation is the addition of a methyl (CH3) group at a CpG site; where Cytosine and Guanine are in
sequence with a phosphate bond between them, on a single DNA strand
– The methyl group is added using an enzyme called DNA methyltransferase
...
This
opens the structure and activates the chromatin , allowing genes to be transcribed
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Depending on the
position of the lysine, it can cause the inactivation of DNA(makes It difficult for transcription factors to
attach to promoter regions) or the activation of a region
...
It can do this by
blocking sites at mRNA which silences genes and causes the mRNA strand to degrade
– An example is Micro RNA (miRNA) which forms complementary pairings with targeted sections of mRNA

Cystic Fibrosis
Cystic Fibrosis is an autosomal recessive disorder that affects the CFTR gene on the 7th chromosome and the CFTR
protein - mainly affecting the digestive and respiratory systems
–
–

People with Cystic Fibrosis lose large amounts of salt when they sweat, affecting the balance of minerals in
the blood which may lead to abnormal heart rhythms
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Autosomal recessive inheritance - homozygous recessive; the presence of two mutant genes is needed for cystic
fibrosis to appear
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Why is cystic fibrosis a genetic disease?
– In some individuals the genes undergo a mutation (base pairs in genes
are altered), this can cause the body to make the incorrect protein or
no protein at all
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– In order for a child to have cystic fibrosis, they have to inherit two
mutated genes - one from each parent
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– This means that different bonds form and the primary structure folds differently, forming a different shaped
protein that doesn't perform the right function
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– The negatively charged chloride ions regulate the movement of water
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– Resulting in thicker and stickier mucus , which is hard for cilia to move from the lungs
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, this is done by antibiotic therapy; kills the
bacteria
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– Chest therapy is also done to manage lung problems - this is when mucus is drained from the lungs
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– Physiotherapy - dislodges muscle mucus, allowing more efficient gas exchange

Genetic Screening/Testing
Genetic screening - using diagnostic tests to identify those what are at a high risk at passing or having a specific
genetic disorder
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A sample of tissue is obtained from the
placenta
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Centrifuging is
performed on the tissue and then karyotyping - results take 5 - 10 days
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Amniotic fluid is removed using a needle and syringe
that is passed through the abdomen
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5 - 1% chance of miscarriage (other factors
may contribute)
1% chance of false negative/positives

Ethical Issues
Benefits
– The identification of treatable genetic disorders at an early stage
– Allows couples to make informed decisions about parenthood - don't have to terminate a pregnancy if it
comes to it
– Allows people at risk to take appropriate preventive measures e
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no smoking - genetic susceptibility is still
difficult to evaluate
Issues
–
–
–
–
–
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Can increase personal anxieties about health
Difficult to decide whether to pass on genetic information to other family members
The decision to terminate a pregnancy - the susceptibility (prediction) may not be the most accurate
Misuse of genetic screening in employment or insurance - may require genetic tests before obtaining
insurance (individuals may be taken advantage of)
Some individuals may not disclose results to other family members

Genetic Mutations
Mutation - sudden inheritable change in the sequence of nucleotides in DNA, these are due to a point mutation and
occasional errors in transcription and DNA replication
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Mutations can cause the sequence of amino acids in the polypeptide chain, this can form no protein or a protein that
performs the wrong function
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– Same-sense mutation/ silent mutation - when a base alteration in a codon doesn't alter the amino acid
sequence - happens because genetic code is degenerate
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CAUSES
Spontaneous mutation - a result of errors in the replication of DNA, enzymes make rare ,mistakes that cause base
changes
Induced mutation - changes in DNA due to mutagens (radiation or chemical) causing genes to mutate at a faster rate
Examples of induced mutations:
– Ionising radiations - e
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X-rays, cosmic rays, alpha and beta rays and electromagnetic gamma rays
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– Non-ionising radiations - e
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UV light of wavelengths 250 - 270 nanometers, this is absorbed by the
nitrogenous bases in DNA and modifying them e
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thymine dimers are produced (adjacent thymine bases in
a DNA join together rather than with adenine)
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– Chemicals - e
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mustard gas, this adds methyl or similar alkyl group to guanine causing it to be unable to
pair, so guanine is released from the DNA and another base moves in instead
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– Sickle cell anemia the base T is substituted with base A(GAG to GTG), forming a different complimentary
codon (GUA instead of GAA), so the amino acid valine is present instead of glutamic acid, forming
hemoglobin-S
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– The promoter region is the start of the region that codes for the RNA molecule, the terminator region is the
end
...
This causes the DNA
double-helix to unwind
...

– The DNA sequence where the protein is being coded is the sense strand and the site at which the mRNA
forms is the anti-sense strand
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(Adenine with Uracil and Guanine with
Cytosine)
...

– Once the RNA polymerase reaches the terminator region, the (messenger)mRNA (always codes 5’ to 3’) is
formed
...

– The mRNA strand has exon regions, these code for a protein, as well as intron regions which are non-coding
section
...

– The intron regions are removed through a process called intron splicing and is performed by a complex of
proteins and RNA called a spliceosome
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TRANSLATION – occurs at the ribosomes in the cytoplasm
– The mRNA molecule moves into the cytoplasm for translation to occur
– The nitrogenous bases are grouped into the three letter groups called codons, these code for a specific
amino acid, or AUG that codes for a start codon and UGA, UAG and UAA that code for a stop codon
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– The type of amino acid is determined by the anticodon(complimentary triplet codon to the codon on mRNA)
sequence on the tRNA
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– Another tRNA molecule then attaches to the next triplet codon
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– The ribosome moves along, and the process continues until a stop codon is reached and it detaches
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The polypeptide chain then folds to form a new protein
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Gene - a sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide
chain
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Degenerative - most amino acids are encoded by more than one codon, this decreases the chances of
mutations
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Conservative hypothesis – suggests the original double helix remain intact and, in some way, instructs the
formation of a new, identical double helix made up of entirely new mononucleotides
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The enzyme DNA helicase breaks the hydrogen bonds formed between the nitrogenous bases on the two
polynucleotides, causing the DNA strands to unwind and form the replication fork
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The two DNA strands are then kept apart by single-strand binding (SSB) proteins; the two strands are going
to want to form hydrogen bonds again after being separated – so this protein is essential
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Once the DNA strands have been separated, DNA primase generates a short piece of RNA called a RNA
primer
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4
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5
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This is the leading strand and is built towards the replication fork
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Since DNA molecules have anti-parallel strands, the other strand is in the opposite direction(5’ to 3’)
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7
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8
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9
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10
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11
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12
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MESLESON AND STAHL
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–
–
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Meselson and Stahl showed that DNA is replicated using the semi-conservative method
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Two samples of E
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The other one was grown in 15N, so over time the bacteria would incorporate 15N atoms into its
DNA, so this bacterium is denser than the one grown in 14N
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The DNA from the
denser nitrogen settled lower in the centrifuge tube
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Another DNA sample was extracted and spun in a centrifuge
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The Structure of Mononucleotides
MONONUCLEOTIDE
A mononucleotide is a biological molecule made up of a pentode
sugar (5 carbon atoms), a nitrogen-containing base and a
phosphate group
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The four possible bases are Adenine
(A), Thymine(T), Cytosine(C) and Guanine (G)
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The
four possible bases are Adenine (A), Uracil(U), Cytosine(C) and Guanine (G); uracil replaces thymine
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The phosphate group (2 hydrogen atoms, 1 phosphate and 4 oxygens)
joins to the pentose sugar at carbon 5; a condensation reaction occurs
that results in the release of a water molecule and forming a
phosphoester bond
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So another condensation reaction
occurs, resulting in the release of water
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POLYNUCLEOTIDES
When many mononucleotides join, they from a polynucleotide
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RNA is made up of one polynucleotide stand and DNA is made up of two (double-helix structure)
More nucleotides join to form more phosphodiester bonds, creating a polynucleotide chain
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therefore the amounts in a DNA molecule are always equal
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A DNA molecule has a 5 prime, 3 prime, the 5 prime refers to the top end in which the 5 th carbon on the
pentose sugar isn’t bonded and the 3 prime refers to the 3rd carbon on the pentose sugar that isn’t
bonded
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It also contains
nuclear pores that allow molecules through
– Function - it controls the cells activities
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Endoplasmic Reticulum (ER) - membrane bound
– Structure - found near the nucleus and made up of flattened sacs called cisternae - continuous with the nuclear
envelope
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The rough ER has many ribosomes on its outer surface
and the smooth ER has no ribosomes
...

Golgi Apparatus - membrane bound
– Structure - a group of fluid-filled, membrane bound flattened sacs
...
It also makes
lysosomes
Lysosomes - membrane bound

–
–

Structure - round organelles that are membrane bound (surrounded by a membrane)
Function - contain digestive enzymes that are used to breakdown materials or digest organelles that are worn
out or destroyed
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Their enzymes are released and
destroy the contents of the cell - apoptosis (programmed cell death)

Mitochondria - membrane bound
– Structure - double membrane, the inner one is folded to from cristae that are folded in the matrix - increasing
surface area
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– Function - the thylakoids contain chlorophyll that absorb light and allow photosynthesis to take place
Vacuoles - membrane bound
– Structure - large vesicles, formed by the joining of many vesicles that contain water with different compounds
– Function - in plant cells they are important in maintaining a turgor pressure

Ribosomes - non-membrane bound
– Structure - small spherical organelles composed of two subunits(large and small) and made of ribosomal RNA
and protein
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– Eukaryotic cells - main type is 80S(svedberg - unit to measure rate of particles settling at the bottom of a
centrifuge) ribosomes
...
Made up of a 50S large
subunit and a 30S small subunit and ratio of RNA:Proteins is 2:1
– Function - translate genetic information in the form of mRNA to form proteins
Centriole - non-membrane bound
– Structure - made of microtubules, found next to the nucleus of animal cells
– Function - move chromosomes around by forming spindle fibers during cell division
Cell Wall - non-membrane bound
– Structure - usually contains three layers and is mostly made up of cellulose
– Function - provides tensile strength and protection against osmotic stress
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–

Prokaryotic Cells – contain no double-membrane bound organelles
Flagellum - long hair-like structure that rotates to make the cell move, not all
prokaryotic cells have them and some may have more than one
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Contain genes for antibiotic resistance that can be passed between
prokaryotes
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Some believe they play a
role in cellular processes and some think they are just artefacts produced
when preparing the cell for viewing by a electron microscope
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Maintains shape
and support and prevents swelling and bursting
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However they make bacteria more vulnerable to viruses as bacteriophage uses pili as an entrance to the cell
COMPARISON
Eukaryotic Cells

Prokaryotic Cells

– Cells contain a nucleus
– DNA is associated with proteins and is in the
–
–

–
–

nucleus
Contain discrete membrane-bound organelles
e
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mitochondria, the nucleus and Golgi
apparatus
These cells are larger diameter of 20 nano
meters or more

–
–
–
–

– Not all contain a cell wall
– Examples are bacteria

Cells don't have a nucleus
DNA isn't associated with proteins, free in the
cytoplasm
Don’t contain any membrane-bound organelles
Most are extremely small - diameters that are
between 0 and 5 nanometers
All have a cell wall
Examples are animals, plants, fungi and
Protoctista

Gram Positive and Gram Negative Bacteria

Gram Positive bacteria contain a thick cell wall made of the substance peptidoglycan (a macromolecule composed of
sugars and amino acids) containing substances like teichoic acid
...

The thick layers help support the cell membrane and allow other molecules to attach
Gram Negative bacteria consists of a single thin layer of peptidoglycan with no teichoic acid between the two layers
...
This bacteria Is harder to kill as the outer lipid membrane is more difficult to penetrate than the
peptidoglycan layer, offering it more protection

Light and Electron Microscopes
Actual Size = Image Size/Magnification
Robert Hooke - English scientist, best known for his law of elasticity (Hooke's Law)
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004nm wavelength) and uses magnets instead of light to focus the beam
onto the object
...
These images can
later be converted to colour using computer software's
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It produces a magnified 2D image of x 500 000
• Scanning Electron Microscope (SEM) - electrons don’t pass through the object, but bounce off it
instead, this creates a 3D image with a magnification of x 100 000
Advantages

– Resolution is 0
...
The RER then stores and transports the proteins within the cell after they're made
Cells that produce hormones and digestive enzymes have large amounts of RER as they need to be able to
secrete proteins without affecting the cells activities - exocytosis
Smooth endoplasmic reticulum (SER) - not covered by ribosomes, involved in the synthesis and transport of
steroids and lipids
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