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A gene is a heritable factor that consist of a length of DNA and
influences a specific characteristic
There are 46 DNA molecules in a human cell; but there are thousands of
genes; each gene consists of a much shorter length of DNA than a
chromosome and that each chromosome carries many genes
A gene occupies a specific position on one type of chromosome
Genes are linked in groups and each group corresponds to one of the type of
chromosomes in the species ‘
Each gene occupies a specific position on the type of chromosome where it is
located; its called the locus of the gene
The various specific forms of a gene are alleles
Mendel deduced that differences between varieties that he crossed together
occurred due to different heritable factors
Pairs of heritable factors are alternative forms of the same gene
These different forms are called alleles; there can be more than two alleles
Sometimes, there can be a large number of different alleles for a specific
gene
As alleles are different forms of the same gene, they occupy the same
position on one type of chromosome—same locus; only one allele can
occupy the locus of the gene on chromosome
Most plants and animals have two copies of each type of chromosome
Alleles differ from each other by one or a few bases only
A gene consists of a length of DNA, the base sequence can be hundreds or
thousands of bases long; the different alleles have slight variations in the
sequence
Positions in a gene where more than one base may be present are called
single nucleotide polymorphins (SNPs) and its pronounced snips

Several snips can be present in a gene, but even then alleles of the gene
differ only slightly

Mutations and Genomes

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New alleles are formed by mutation
New alleles are formed from other alleles by gene mutation; mutations
are random changes and there is no method for a particular one to be
carried out
Most significant kind is a base substitution
One base is replaced by a different one
A random change to an allele that developed by evolution over millions of
years is unlikely to be beneficial; almost all mutations are neutral or
harmful
Some are lethal; they can cause death of the cell in which the mutation
occurs
Mutations in cells that develop into gametes can be passed on to offspring
and cause diseases
The genome is the whole of the genetic information of an
organism
Genetic information is contained in the DNA, so a living organism’s
genome is the entire base sequence of each of its DNA molecules
In humans, the genome consists of 46 molecules that ofrm the
chromosomes in the nucleus plus the DNA molecule in the mitochondrion;
similar pattern in other animals
In plants, the genome is the DNA molecules of the chromosome plus the
DNA molecules in the mitochondrion and chloroplast
The genome of prokaryotes is much smaller and consists of the DNA in a
circular chromosome, plus any plasmids that are present
The entire base sequence of human genes was sequenced in the
Human Genome Project

The project began in 1990 and was aimed at finding the base sequence of
the entire human genome; it drove improvements in sequence techniques
which allowed a draft sequence to be published much sooner than
expected (2000)
The final sequence was completed in 2003
The knowledge of the entire base sequence hasn’t given us immediate
understanding; it has given us a mine of data, which will be worked by
researches
It is possible to predict which base sequences are protein coding genes;
there are 23,000 of these in the human genome
Estimates for the number of genes were much higher
Another discovery was that most of the genome is not transcribed;
originally called Junk DNA it is being increasingly recognized that in these
regions there are elements that affect gene expression along with satellite
DNA
The genome sequences isn’t THE human genome, it is A human genome
Work is being done to discover the variations in sequences between
humans

Prokaryotes and Chromosomes

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It is the commonest genetic disease in the world; occurs due to a
mutation of the gene that codes for alpha globin polypeptide in
hemoglobin (Hb)
Most humans have Hb(A); if a base sub at the sixth codon makes it GTG
from GAG; then Hb(S), the new allele is formed; the offspring only

inherits it if it occurs in the cell of an ovary or testis that makes a sperm
or egg

When the Hb(S) allele is transcribed, the mRNA produced has GUG as its
sixth codon instead of GAG, and when this mRNA is transcribed, the sixth
amino acid is valine instead of glutamic
This causes hemoglobin molecules to stick together in tissues with low
oxygen concentration
The bundles of hemoglobin that are formed are rigid enough to distort the
blood cells into a sickle shape
These sickle cells damage the tissue by becoming trapped in capillaries
and blocking them; when sicle cells return to high oxygen concentrations,
the bundle breaks up
Thesechanges occur over and over and the both the hemoglobin and
plasma membrane are damaged and the life of a red blood can be
shortened
Body can’t replace fast enough, and so anemia occurs

Prokaryotes have one chromosome consisting of a circular DNA
molecule
In most prokaryotes there is one chromosome, made of circular DNA
containing all the genes for life processes
The DNA in bacteria isn’t associated with any proteins, so its naked
There is usually only one copy of each gene; two identical copies are
briefly present after the replication, but that is in preparation for division
Some prokaryotes also have plasmids, but eukaryotes do not

Plasmids are small extra DNA molecules that are commonly found in
prokaryotes but are very unusual in eukaryotes
They are small, circular, and naked with few genes useful for life
processes; genes for antibiotic resistance are present in plasmids; only
beneficial when there is a antibiotic present, but not at other times
Plasmids aren’t always replicated at the same time as chromosomes;
there can be multiple plasmids in a cell and a plasmid may not even
passed to the split cell
Copies of plasmids can be transferred to spread amongst a population; its
even possible for a plasmid to cross the species barrier; plasmids are
used to transfer genes between species artificially

Measuring length of DNA Molecules

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Technique for producing images of DNA molecules for E
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Nuclei

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Haploid nuclei have one chromosome of each pair
A haploid nucleus has one chromosome of each type; it has one full set of
chromosome that are found in its species
Gametes are sex cells that fuse together during reproduction ; gametes
have haploid nuclei so in humans both egg and sperm cells have 23
chromosomes
Diploid nuclei have pairs of homologous chromosomes
Ad diploid has two chromosomes of each type; two full sets of
chromosome that are found in the species (in humans 46)
When haploid gametes fuse, the zygot with a diploid nucleus forms; when
it divides by mitosis more cells with diploid nuclei are produced
Diploid nuclei have two copies of every gene, apart from genes on the sex
chromosomes
Effects of harmful recessive mutations can be avoided if a dominant allele
is also present; organism are often more vigorous if they have two
different alleles of genes instead of one
The number of chromosomes is a characteristic feature of
members of a species
Organisms with a different number of chromosomes are unlikely to
interbreed
The number of chromosomes can change during evolution of a species; it
can decrease if chromosomes fuse or increase if splits occur
There are also mechanisms that can cause the chromosome number to
double; however these are rare and chromosomes can remain unchanged
for millions of years

Meiosis

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Sex is determined by sex chromosomes and autosome are
chromosome that don’t determine sex
X chromosome is relatively large and has its centromere near the middle;
the Y chromosome is much smaller and has its centromere near the end
X chromosome has many genes essential in both males and females; all
humans have X chromosome; Y chromosome only has a small number of
genes; a small part of the Y chromosome has the same sequence as the X
One Y chromosome gene in particular causes a fetus to develop as a
male; it is called either SRY or TDF; initiates development of male
features, testes and testosterone
Females have two X chromosomes; they pass on one of their X in each
egg cell so all offspring inherit an X from their mother; gender is

determined at the moment of fertilization by one chromosome carried in
the sperm; can either by X or a Y
A karyogram shows the chromosome of an organism in
homologous pairs of decreasing length
The chromosomes of an organism are visible in cells that are in mitosis
with cells in metaphase giving the clearest view
If dividing cells are stained and placed on a microscopic slide and are then
burst by pressing on the slip, the chromosomes spread
Then a micrograph can be taken of the stained chromosomes
One diploid nucleus divides by meiosis to produce four haploid
nuclei
The nucleus divides twice; first division produces two nuclei, each of
which divides again to give four total nuclei
Two divisions: Meiosis I and Meiosis II
The nucleus that undergoes the first division of meiosis is diploid;
contains two chromosomes of each type
Each of the four produced have one chromosomes of each type; they’re
haploid
Meiosis is also known as reduction division
Cells produced by meiosis I have one chromosome of each type; so the
halving of the chromosome number happens in the first division not the
second
Two nuclei produced in meiosis I have the haploid number of
chromosomes, but each chromosome still consists of two chromatids

These chromatids separate during meiosis II, producing four nuclei that
have the haploid number of chromosomes, and only one chromatid
The halving of the chromosome allows a sexual life sycle with
fusion of gametes
In an asexual life cycle, offspring have same chromosomes as the parent
thus they are identical
In a sexual life cycle, there is a genetic diversity as there are
chromosome differences
In eukaryotic organisms, sexual reproduction involves process of
fertilization; it’s the union of sex cells, gametes, from two parents;
Meiosis can happen at any stage during sexual life cycle, but in animals it
happens during the process of creating gametes; body cells are therefore
diploid and have two copies

DNA and Bivalents

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DNA is replicated before meiosis so that all chromosomes consist
of two sister chromatids
During early stages of meiosis, chromosomes shorten by supercoiling; as
they become visible, its clear that each chromosome consists of two
chromatids
Initially, the two chromatids that are part of the chromosome are
genetically identical; the replication is accurate
DNA is not replicated between first and second division of meiosis; that is
why the four product cells only have chromatid each
The early stages of meiosis involve pairing of homologous
chromosomes and crossing over followed by condensation
Homologous chromosomes pair up
DNA replication has already occurred, thus, each chromosome consists of
two chromatids and so there are four DNA molecules in a pair of
homologous chromosomes
A pair of homologous chromosomes is bivalent and pairing process may
be called synapsis
Soon after synapsis, a process called crossing over takes place; the
outcome is the creation of a junction where one chromatid in each of the
homologous chromosomes breaks and rejoins with the other chromatid
This can occur at random positions along chromosomes; at least one
crossover occurs in each bivalent and there can be several
The crossover occurs at precisely the same position on two chromosomes
involved, so there is an exchange of genes between the chromatids; since

chromatids are homologous yet not identical, some alleles of the
exchanged genes can be different
Thus, chromatids with new combinations of alleles are produced
Orientation of pairs of homologous chromosomes prior to
separation is random
When homologous chromosomes are condensing inside the nucleus;
spindle microtubules grow from poles of the cell
After nuclear membrane has broken down, these microtubules attach to
the centromeres of the chromosomes
The attachment is not the same as mitosis:
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Each chromosomes is attached to one pole only, not both
Two homologous chromosomes in a bivalent are not attached to
the same pole



Attachment to which pole is based on which way the pair of
chromosomes is facing; this is orientation
Orientation is random, so each chromosome has the equal
chance to each pole
Orientation of one bivalent doesn’t affect other bivalents




Separation of pairs of homologous chromosomes in the first
division of meiosis halves the chromosome number
Movement of the chromosomes is not the same in meiosis as in mitosis
during first division;
In mitosis the centromere divides and two chromatids move to opposite
sides, but in meiosis the entire chromosome moves to the pole, no
division
The two chromosomes in each bivalent are held together by chiasmata,
but these slide to the end of the chromosomes and allow for separation

The separation of a bivalent is called disjunction
Disjunction results in halving the chromosome number per cell; it is the
first division of meiosis that is the reduction division
The two cells after meiosis I are haploid

Obtaining cells from a fetus

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Amniocentesis: needle is passed through the mother’s abdomen wall;
ultrasound guides needle; sample of amniotic fluid is withdrawn from
amniotic sac; fluid contains fetal cells; chance of miscarriage 1%
Chronic villus: sampling tool enters vagina and obtains cells from
chorion; chorion is membrane where placenta develops; done in earlier
stage than amniocentesis; chance of miscarriage 2%

Stages of Meiosis

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Prophase I: cell has double chromatid; homologous chromosomes pair up
(synapsis); crossing over takes place
Metaphase I: spindle microtubules move homologous pairs to equator of
cell; random orientation
Anaphase I: Homologous pairs are separated; one chromosome of each
pair moves to each pole (disjunction)
Telophase I: chromosome uncoil; during interphase that follows, no
replication occurs; reduction of chromosome number from diploid to
haploid; cytokinesis
Prophase II: Chromosomes, sstill have two chromatids, condense and
become visible
Metaphase II: microtubules attach just as in mitosis
Anaphase II: centromeres separate and chromatids are moved to
opposite poles
Telophase II: chromatids are at opposite poles; nuclear envelope forms;
cytokinesis occurs

Gregor Mendel and Heredity

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Crossing over and random orientation promotes genetic variation
Apart from X and Y chromosomes; each gene is available in two copies; in
some copies, these genes are the same allele and there will be one copy
of that allele in each gamete
Its likely to have several thousands of genes that have different alleles;
each of the alleles has an equal chance of being passed down
Random orientation of bivalents:
 In Metaphase I; the orientation of bivalents is random and the
orientation of one bivalent doesn’t influence the orientation of
the others
 Leads to genetic variation among genes that are on different
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chromosome types
For every additional bivalent, number of chromosome
combinations in a cell made by meiosis doubles
Humans have over 8 million combinations
For a hapoid number of n, the number of possible combinations
is 2^n

Crossing over:
 Without crossing over, combinations of alleles on chromosomes
would be forever linked together
 Increases number of allele combinations possible generated to a
point of pretty much infinite
Fusion of gametes from different parents promotes genetic
variation
Gamete fusion:
Allows alleles from different individuals to be combined in one
The combination is unlikely to have existed before
Promotes genetic variation

Genetic variation is essential for evolution

Mendel discovered principles of heredity by studying 7 characteristics of
pea plants; these characteristics didn’t blend, and they were passed down
in mathematical ratios
These hereditary factors (now called genes) determine characteristics,
these factors occur in duplicate in parents, during gamete formation these
two copies separate
Mendel’s experiments worked due to a few things:
 each of the characteristics were determined by one gene rather
than several


the characteristics he was looking at were all on different
chromosomes

Initially investigated single contrasting characteristic (monohybrid cross)
Began investigation with true bred plants, so all the generations were
only purely tall or short
After cross fertilization of purebreds:
 Generation F1: all tall
 Generation F2: ratio of 3 tall to 1 dwarf
Since dwarfness disappeared then reappeared, he concluded that it was
preserved but didn’t show up in presence of tall factor
Tall is dominant; short is recessive
Conclusions from monohybrid cross:
 There are factors that control characteristics
 There are two factors in each cell
 One comes from each parent
 Factors separate during reproduction and either can be passed
on
 Factor of “tall” is alternate for “dwarf”



Factor of “tall” is dominant over “dwarf”

Mendel’s First Law (aka
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4% of females
The female may be:
 homozygous for normal vision (X^B and X^B)



heterozygous (X^B and X^b) and still have normal vision
the only way the female is red green color blind is if she has
homozygous blindness (X^b and X^b)

The male will be red green color blind if he receives a recessive allele
(X^b and Y)
The only way there is a female with red green color blindness is if the
father has the condition and the mother has or is a carrier of it
Hemophilia
Hemophilia is when there is uncontrollable bleeding due to the lack of a
blood clotting mechanism
There are two kinds of hemophilia, A and B
Occurs due to a failure to make an adequate amount of blood proteins
that are essential to the complex blood clotting mechanism
Today, you can just administer the clotting factor that the patient lacks
The proteins production genes are on the X chromosome; it is recessive
allele
Occurs largely in males, due to same reason as red green color blindness
A female may only have it if she homozygous for hemophilia and this is
fatal in the uterus, leading to natural abortion

Genetic Disease and Inheritance

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Most genetic diseases arise from mutations involving a single gene
About 4000 disorders have genetic basis, and they affect 1% -2% of the
population
More than half the known genetic diseases are due to a mutant allele that
is recessive and in this situation the individual is homozygous for a
mutant gene
Common genetic diseases include sickle cell, duchene muscular
dystrophy, severe combined immunodeficiency (SCID), familial
hypercholesterolemia, hemophilia, thalassemia, and cystic fibrosis
Cystic fibrosis is the most common genetic disease in Caucasians
 Occurs due to mutation in chromosome 7
 Affects the epithelial cells
 The normal CF gene codes for a protein that acts as an ion pump
 The pump transports chloride ions across membrane and water



follows the ions, to keep epithelia smooth and moist
The mutated gene doesn’t code for the protein or codes for a
faulty protein
Outcome is that there is dryness and sticky, thick mucus buildup
in epithelia
o Pancreas—secretion of digestive juices is interrupted by
blocked ducts
o Sweat glands, salty sweat is formed
o In lungs where thick mucus builds up; can be fatal
o In reproductive organs

Small percent are caused by dominant allele- such as Huntington’s
disease, due to autosomal dominant allele on chromosome 4
 Extremely rare
 Appearance of symptoms delayed for 40-50 years; by then
could’ve passed genes to children
 Disease takes form in progressive mental deterioration;
accompanied by involuntary muscle movements

Gene Methods

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Abrupt change in structure, arrangement or amount of DNA of
chromosomes
Mutation result in alteration or non production of cell protein; many of
these mutation are neutral but some are lethal or fatal
Mutations that occur in the body cells are called somatic mutations
 Only passed down to immediate descendants of that cell
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


Can cause cancer
These occur with increasing frequency as an organism ages
Cancers are most common in elder rather than younger
Mutations are eliminated when the cells die

Mutations that occur in the cells of the gonads (germ line mutations) give
rise to gametes that have an altered genome
 Inheritable; lead to genetic diseases in offspring
 Can occur spontaneously due to error in replication
 Can be caused by mutagens (environmental factors)
o
o
o
o

Includes ionizing radiation, cosmic rays and other radiation
Can lead to break up of DNA molecule
Non ionizing mutagens include UV rays and carcinogens
These act as modifiers of base pairs

Gel electrophoresis is used to separate proteins or fragments of
DNA according to size
Involves separating charged molecules in an electric field, according to
size and charge
Samples placed in wells in gel, gel is put in conducting fluid, electric field
is appliedcharged molecules move through gel, opposite charges move
in opposite directions since proteins can be positive or negatively charged
Gel used consists of a mesh of filaments that resist movement of
molecules

DNA molecules from eukaryotes are too long to move through the gel, so
they must be broken down into smaller fragments
DNA molecules have negative charge and move same direction but not
same rate; small fragments move faster than large fragments, so they
move further
PCR can be used to amplify small amounts of DNA
The polymerase chain reaction is used to make large numbers of copies of
DNA
In theory, just need a single molecule to start PCR
Makes it possible to study DNA further without the risk of using up a
limited sample (eg: DNA from fossil)
Small amounts of DNA can be used in forensic investigations
PCR is used to copy specific DNA sequences; sequence is selected for
copying by using a primer that binds to the start of the desire sequence;
primer binds by complementary base pairing
Selectivity allows particular desire sequences to be copied from a whole
genome or even a greater mix of DNA
You can use PCR to detect genetically modified DNA, as the primer would
to it and amplify it; if isn’t present, then nothing will happen
DNA profiling involves comparison of DNA
Stages of DNA profiling
 Sample of DNA is obtained
 Sequences in DNA that vary largely between individuals are
selected and amplified
 Copied DNA is split into fragments through restriction
endonucleases
 Gel electrophoresis is run to separate




Produces pattern of bands
Profiles of different individuals can be compared to which bands
are the same and which aren’t

Genetic Modification

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Genetic modification is carried out by gene transfer between
species
Genetic code is universal, so when genes are transferred between
species, the amino acid sequence is unchanged--- same polypeptide
produced
Genes have been transferred from eukaryotes to bacteria; one of the
early examples was the transfer of the gene for making human insulin to
a bacterium
New characteristics can be added to animal species; goats have been
produced that secrete milk containing spider silk protein; spider silk is
very strong but spiders could not be used to produce it commercially
Crop plant can be made too; genes from snapdragons have been
transferred to tomatoes to make them purple rather than red
Golden rice needed two genes from daffodils and one from a bacterium to
make beta carotene

Techniques for Gene Transfer to Bacteria

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Plasmids
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

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Plasmid is a small extra circle of DNA; smallest plasmids have a
thousand base pairs, but they can have over a million base pairs
occur commonly in bacteria
most abundant plasmids are those with genes that encourage
their replication in cytoplasm and transfer from one bacterium to
another
Plasmids are not pathogenic and natural selection favors
plasmids that confer an advantage on a bacterium than a
disadvantage
Used for exchanging genes, so they are absorbed and
incorporated into main circular DNA

Restriction Enzymes
 Restriction enzymes (endonucleases) are enzymes that cut DNA
molecules at specific base sequences; can be used to cut open
plasmids and also cut out desired genes from larger DNA



molecules
Can cut two strands of a DNA at different points; leaves single
stranded section called sticky ends
These sticky ends have complementary base sequences so can
be used to link together pieces of DNA, by hydrogen bonding

DNA Ligase
 DNA ligase is an enzyme that joins DNA molecules firmly by
making sugar phosphate bonds between nucleotides


DNA ligase is used to seal up nicks in sugar phosphate backbone
once the desire gene is put into the stick ends

DNA is copied to create mRNA which is copied again to make cDNA, a
copy of the original DNA

Clones

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Clones are a group of genetically identical organisms, derived
from a single original parent cell
A pair of identical twins is the smallest clone that can exist; either result
of human zygote dividing into two two cells, which each develop into
separate embryos, or splitting of an embryo into two parts which develop
into separate individuals
Identical twins are not fully identical, they can have different finger
prints; better term is monozygotic
A clone can exist of very large numbers of organisms

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