Text: Chapter 14
Human Biology Stage 3
Keywords
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Chromosomes
Chromatin
Genes
Somatic cells
 Mitosis
Gametes
 Meiosis
Homologous chromosomes
Locus (loci pl.)
Alleles
Multiple alleles
 Codominance
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Polymorphism
Monogenic inheritance
Polygenic inheritance
Structural gene
Epigenetics
 Gene regulation
 Gene expression
 Histone proteins
 Methylation
 Transcriptional repression
 Gene silencing
Assumed prior knowledge
 During formation of the gametes,
each gamete only receives one set of
genes. Gametes unite at fertilization
allowing a different combination of
genes. This is called the law of
independent assortment
 Chromosomes and genes both
occur in pairs in the zygote and in all
somatic cells
 Chromosomes are segregated during
meiosis, and only one member of
each pair normally entered a gamete
 Chromosomes and genes both
maintained their individuality
during segregation and each pair
http://highered.mcgrawsegregated independently of each hill.com/sites/0073212040/student_view0/c
other
hapter11/elearning.html
Assumed Prior Knowledge
 The genes in a particular
chromosome are all linked
together to form a set, a
situation called gene
linkage
 The chromosomes of a pair
contain genes that control
the same characteristics.
They are referred to as
homologous
chromosomes
Chromosome
Map of chromosome 7
ncbi.nlm.nih.gov
Chromosomes
 The genes for a particular
www.nwcreation.net
trait occur at a particular
point in the chromosome
called a locus
 There are 2 alternative
forms of a gene called
alleles
 Sometimes there are more
than 2 alleles for a
particular characteristic,
called multiple alleles
and the locus of that gene
is refereed to as multiallelic
Multiple alleles
 The ABO blood group
system is based on the fact
that an individual can possess
any two of three alternative
alleles: Ia, Ib, i
 These loci are found on the
long arm of chromosome 9
 These alleles are responsible
for two different protein
antigens that are found on
the membranes of
eurythrocytes (RBC)
 Ia allele produces antigen a,
allele Ib produces antigen b
and i produces no antigens
http://www.teachertube.com/viewVideo.ph
p?video_id=116181
Multiple alleles
 Alleles Ia and Ib are
codominant that means that
neither allele is dominant to the
other and both characteristics
take place.
 i allele is recessive to both Ia and
Ib
• Polymorphic genes are genes
 Situations where multiple alleles
that may exist in 3 or more allelic
are involved are called
forms
polymorphic. Polymorphism is • Multiple alleles result from
where many different physical
different mutations of the same
forms can result from the
gene
various alleles at a single locus
http://www.biologycorner.com/norns/nor
n%20genetics%20multiple.swf
ABO blood group system
ABO blood group system
Monogenic inheritance
 Inheritance of blood
group is dependent on the
one pair of genes
 This is called monogenic
inheritance. It is when
one pair of genes gives
rise to a discrete
characteristic
 The environment does
not affect the expression
of these genes
Polygenic inheritance
 Many characteristics however, are
inherited through multiple pairs
of genes. This is referred to as
polygenic inheritance.
 The group of genes responsible
for the characteristic are called
polygenes
 Polygenes are located at different
loci and may be on different
chromosomes
 The environment can influence
the expressions of these
characteristics.
Example: skin colour, eye colour,
height, weight.
Gene expression
 The process of copying information
from DNA onto messenger RNA and
then translating the message into a
series of amino acids to form a
protein is called gene expression
 They way that a gene is expressed
(genotype) is dependent on several
different factors, including the
environment. An example of this
interaction occurs between the ABO
blood system and the environment.
 Gene expression is also affected by
our diet
 Intellectual development is
influenced by our surroundings
 http://www.ece.iit.edu/~biitcomm/intro/1%20Animations/2%20Gene%20expression/Gene%20expressio
n.swf
Regulation of gene expression
 The portion of a DNA
molecule that contains the
code for manufacture of a
protein is called a
structural gene
 The cell’s control over
when a particular protein
is made and the quantity in
which its made in is called
regulation of gene
expression or gene
regulation
www.docstoc.com -
Epigenetics
 Epigenetics: inherited and
reversible modifications to
neucleotides and
chromosomes that do not
change the sequence but can
alter gene expression
 These modifications can
switch genes on and off and
determine which proteins are
transcribed when
 Gene expression can be
altered by the chemical
modification of DNA by 2
means:
Epigentics 10mins
1. Histone modification
https://www.youtube.com/watch?v=i9a-ru2ES6Y
http://learn.genetics.utah.edu/content/epigenetics/intro
2. DNA methylation
/
http://www.dnatube.com/video/29822/Epigenetics
Histone modification
 Inside the nucleus, the DNA
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molecule is associated with
histone proteins. It forms a
complex called chromatin.
Chromatin sits in the nucleus
like a bowl of spaghetti
When a cell prepares to
divide, the DNA becomes
highly coiled to form
chromosomes.
The histones act as a spool
around which the DNA can
be wound.
The hereditary materials
passed on to the offspring
includes DNA, the histone
proteins and other chemicals
http://www.dnatube.com/video/5631
/Epigenetics-and-Cancer
Histone modification
 Modified histone proteins can change the shape of the chromatin.
 This can determine whether the DNA molecule can be transcribed or
not.
 If chromatin is in a loose form, it is active, and the associated DNA can
be transcribed.
 If chromatin is condensed and in a compact form, then it is inactive,
and DNA transcription does not occur.
DNA methylation
 The best known epigentics
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process is methylation of DNA
This is the addition or removal
of a methyl group (CH3) to the
DNA strand
The methyl always attaches to
the site where there is a cytosine
and a guanine nucleotide next
to each other.
These are known as CpG
islands.
These sites are not uniformly
distributed
DNA methylation
 Methylation of the CpG islands is associated with transcriptional
repression or gene silencing
 Nutrients such as folate & choline (vitamin B group) and methionine
(amino acid) play an important role in the process as they are major
dietary sources of methyl groups
 DNA methylation is a normal part of embryonic cell differentiation and
its influence persists throughout the life of the cell
 These altered methylation patterns can be passed on
http://www.dnatube.co
m/video/5923/Effect-ofmethylation-on-aging
Links
 https://www.youtube.com/watch?v=fMxgkSgZoJs
 Epigentics documentary (50 minutes)
 http://learn.genetics.utah.edu/content/epigenetics/
(great resource!)
 http://learn.genetics.utah.edu/content/epigenetics/tw
ins/
 https://www.youtube.com/watch?v=JTBg6hqeuTg
 Epigenetics Tedx talk 20mins