Download B2 Topic 1 Revision Session

B2 Topic 1
Revision Session
Chromosome
Gene
DNA Helix
Bases
Backbone
Base pairs
DNA Structure Key Points
• Each gene is a length of DNA, DNA is a long
coiled molecule formed from two strands.
The strands are twisted in a double helix.
• The two strands of the double helix are
joined by pairs of bases. There are four
different bases in DNA.
• A= Adenine, T=Thymine, C=Cytosine,
G=Guanine
• Weak hydrogen bonds between the base pairs
hold the DNA strands together.
DNA Discovery-Key Points
• Rosalind Franklin and Maurice Wilkins
worked out that DNA had a helical
structure by directing beams of X-rays
onto crystallised DNA and looking at the
patterns the X-rays formed as they
bounced off.
• James Watson and Francis Crick used
these ideas along with the knowledge that
the amount of adenine +guanine matched
the amount of thymine +cytosine, to make
a model of DNA molecule where all the
pieces fitted together.
Practice Question of DNA
structure and Discover
(b) Describe the structure of DNA,
including the roles of the scientists
involved in its discovery.
(6)
Mark Scheme
• A description including some of the following points in a logical sequence
Points relating to DNA structural features:
·
two strands
·
double helix
·
(contains) bases
·
A, T, C, G
·
adenine / A paired with thymine / T
·
guanine / G paired with cytosine / C
·
hydrogen / H bonds joining bases Contributions from Scientists:
·
X-ray (crystallography) being used
·
to show helical structure
·
to show diameter of molecule
·
how base pairs are arranged was shown
·
how strands are arranged was shown
·
modelling
·
reference to using other people's ideas
(a) Describe how a section of DNA determines the structure of a protein.
(4)
Making a Protein?
(a) Describe how a section of DNA
determines the structure of a protein.
(4)
• A description including the following linked points
· ref to a gene (coding for protein)(1)
· sequence of bases determines sequence of
amino acids (1)
· idea of one code / triplet / codon / 3 bases
(for one amino acid) (1)
· several amino acids make up a protein /
(poly)peptide (1)
· transcription / detail of transcription (1)
· translation / detail of translation (1)
Human Genome Project
• The Idea was to map the 25000 Human
genes!
• Thousands of scientists from all over
the world work together.
• The big idea was to find every single
gene.
• One you find them then you figure out
what they do.
Good Stuff
Predict and prevent diseases
• If doctors knew what genes predisposed people to what diseases, we call
get individually tailored advice on the best diet and lifestyle to avoid are
likely problems.
Develop new and better medicines
Maybe one day we’ll all have the medicines designed especially for us- these
will be based on the way our individual body will react to the disease and
to possible treatments. More generally knowing how a disease affects us
on a molecular level can make it possible to design more effective
treatments.
Accurate diagnosis
Some diseases are hard to test for (e.g. You can only tell for sure if someone
has Alzheimer's after they die), but if we know the genetic cause,
accurate testing will be a lot easier.
Improving forensic science
Forensic scientists can produce a ‘DNA fingerprint’ from biological material
found at a crime science. If this matches your suspects DNA.. He or she
was almost certainly present. In the future it might even be possible to
figure out what the suspect looks like from DNA found at the scene of
the crime.
Crops can be given extra genes for new and useful
characteristics. They are genetically modified (GM).
 pest resistance – wont be damaged by
insects or other pests
 frost resistance – wont be damaged by the
cold
 disease resistance – wont get as many
diseases
 herbicide resistance – wont get damaged by
chemicals
 drought resistance – dont need as much
water
 longer shelf life – wont go off as quickly
Producing insulin with bacteria
Why are enzymes so specific?
Enzymes are very specific about which reactions they catalyze.
Only molecules with exactly the right shape will bind to the
enzyme and react. These are the reactant, or substrate,
molecules.
The part of the enzyme to
which the reactant binds is
called the active site.
This is a very specific shape
and the most important part of
the enzyme.
What happens at the active site?
In the same way that a key fits into a lock, so a substrate is
thought to fit into an enzyme’s active site. The enzyme is the
lock, and the reactant is the key.
+
enzyme
+
reactant
↔
↔
enzyme-reactant
complex
↔
↔
+
enzyme
+
products
The lock and key model
• Carbohydrase or Amylase
enzymes break down starch into
sugar
• Protease enzymes break down
proteins into amino acids
• Lipase enzymes break down fats
into fatty acids and glycerol.
Mitosis and Meiosis Compared
Mitosis
Meiosis
Purpose
To make daughter cells
identical to the parent cells
- eg during growth and
repair
To produce sex cells
(gametes)
Takes place ..
In all cells apart from
gametes
In the reproductive organs
(ovaries and testes)
Produces how many
cells?
Two daughter cells
Four gametes
What happens to
number of
chromosomes?
Same number as in parent
cell
Diploid = 46 (in pairs)
Half as many as in parent
cell (The original number
of chromosomes is
restored when two
gametes fuse to form a
zygote.)
Haploid = 23 (single)
How do parent and
daughter cells differ
genetically?
Not at all - genetic material
is copied exactly
(replicated)
Contain a mixture of
chromosomes from two
parent gametes - so
cannot be identical
No - they are clones of
each other
Yes - they are genetically
different from each other
because chromosomes
get shuffled up during
division
Variation between
daughter cells?