Transcript Gregor Mendel & DNA structure

Gregor Mendel
Brno, Austria
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One of the first people to
examine the inheritance of traits
across generations by:
Carrying out many experiments breeding of pea plants
• Examining the traits of the
offspring and parents
• Proposing a model of inheritance
• 8 years (1856-1864)
Why Pea plants?
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There were many
pea plants in the
monastery
garden
Many offspring
produced in just
one cross
Fairly quick to
reproduce- don’t
take too long to
mature
How he did it
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Although not a
professional scientist, his
methods were very
scientific:
• Investigated one or two
traits at a time by
deliberately breeding
selected plants
• Always started with purebreeds for the traits he was
investigating
• Careful records of parents in
crosses
• Used statistics to analyse
results
Traits
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Although he
examined one
at a time,
there were 7
different traits
that Mendel
examined over
the 8 years of
his scientific
work
Cross-breeding
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Transferred
pollen from
one purebreed plant to
another
Observed
offspring traits
before
Interbred the
first
generation
offspring
For Example
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Mendel crossed: pure-bred yellow
seed plant x pure-bred green seed
plant and obtained all yellow seed
offspring
So the factor that caused the yellow
colour was more powerful than the
factor for green colour
Mendel said that the yellow factor
"dominated" the green factor
We now recognise that these factors
are alleles of genes with slightly
different instructions on homologous
chromosomes and that one allele is
dominant to the other
The experiment continues
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Mendel then allowed the first generation to
self pollinate:
Gametes
Y
y
Y
YY
Yy
y
Yy
yy
Phenotypic ratio:
3/4 yellow : 1/4
green or 3 yellow : 1
green
The Findings
Gregor observed the following:
1. Each trait is controlled by a pair of
inherited factors (alleles)
2. For each trait individual plants had
two factors which could be
identical or different (homo or
heterozygous). Gregor called
these pure-bred and hybrid.
3. Each factor was a discrete particle
that retained its identify across
generations (didn’t get diluted or
blended)
The Findings
4. The trait shown in the 1st
generation hybrid plants were
assumed to be dominant while the
hidden trait was recessive
5. During gamete formation, each
pair of factors separated to
different gametes – one factor per
gamete
6. In separating, the factors within
the pair behaved independently of
each other
The findings
7. Results of crosses that were
repeated revealed the same
results, regardless of which plant
was used as the male parent and
which was used as the female
parent
Mendel’s Laws
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From these findings, Gregor
devised two laws of inheritance:
• The principle of segregation of
alleles:
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That alleles of parents are separated
into different gametes
• The principle of independent
assortment:
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That alleles behave independently of
one another when separating into
alleles
Rejected & Rediscovered
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When Mendel presented his
findings to the scientific community
in Brno in 1865-6 they were largely
ignored
It was not until the early 1900s
that they were rediscovered by 3
geneticists working independently
of one another
Not so simple…
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It was found that Mendel’s laws of
inheritance were largely correct
and seemed to apply to many traits
across many different species
However, in the early 1900s two
geneticists discovered that some
traits can be inherited together,
going against Mendel’s laws of
independent assortment (linkage)
Chromosomes
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In 1902, the structure of
chromosomes were first
discovered by an American
scientist
He assumed that these
chromosomes housed the genetic
factors proposed by Mendel
DNA structure
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The 3D structure of DNA was
proposed by Watson and Crick in
1953
What does DNA stand for?
De oxy-ribose
Type of
monosaccharide
Nucleic
Found in the
nucleus
(eukaryote) or
nucleoid
(prokaryote) of
cells
Acid
As in acids
and bases
DNA can also be written as Deoxyribonucleic Acid
Building blocks
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A single unit of DNA is called a
nucleotide it is made up of:
• A Sugar Molecule (Deoxy-ribose)
• A Phosphate molecule
• A Nitrogenous Base
P
P
Building blocks
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P
P
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P
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P
The nucleotides are
bound together via
covalent bonds
between the
phosphate groups,
forming a strand of
DNA
These covalent
bonds are very
strong and difficult
to break
The phosphate and
sugar ‘backbone’
are on the outer
part of the double
helix
DNA
DNA is made up of 2 strands.
The strands are held together by weak bonds
between the nitrogen bases which face inwards
The bonding between the strands is made up of
hydrogen bonds which are so weak that the two
strands will easily separate at 100˚C
There are four types of nitrogen bases:
Cytosine (C), Guanine (G), Adenine (A) and Thymine (T)
What pairs with C?
What pairs with A?
http://academy.d20.co.edu/kadets/lundberg/dnapic2.html
http://www.tokyo-med.ac.jp/genet/picts/dna.jpg
Complementary Base Pairs
There are four types of nitrogen bases:
Cytosine (C), Guanine (G), Adenine (A) and Thymine (T)
Cytosine pairs with Guanine
These are called
Adenine pairs with thymine
complementary base pairs
http://academy.d20.co.edu/kadets/lundberg/dnapic2.html
http://www.tokyo-med.ac.jp/genet/picts/dna.jpg
DNA
The two strands of DNA run in opposite
directions to one another and are said to be
‘anti-parallel’
Once the 2 strands of DNA are bound together,
the strands coil to form a helical shape
This is why DNA is often called a double helix
In this diagram, the 2 DNA strands have
backbones that are shown in blue and red
The nitrogen bases are shown in yellow
http://academy.d20.co.edu/kadets/lundberg/dnapic2.html
http://www.tokyo-med.ac.jp/genet/picts/dna.jpg
The significance of
complementary base pairing
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Complementary base pairing means
that DNA can act as a template for its
own replication
• If you know the sequence of one chain,
you can infer the sequence of the other
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Contains genetic instructions for
protein production
DNA strands can dissociate and then
reassociate when heated and cooled
respectively
The Gene Code
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The order of the nitrogenous bases A,
T, G & C is very important.
The sequence provides cells with
templates for the production of every
protein in the body
These proteins play many different
roles in the body
Each segment of DNA which
determines the structure of one protein
is called a gene