F334 - The Thread of life

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F334 ­ The Thread of Life:
The structure of DNA:
DNA is a polymer which is formed from many nucleotides.
A DNA nucleotide is made from 2 sugar phosphate backbones, in between which are
nitrogen bases which can hydrogen bond to each other. DNA is double stranded.
The sugar phosphate backbone is made from a deoxyribose sugar attached to a phosphate
group; the two molecules bond together in a condensation reaction.
There are four possible bases in between the two sugar phosphate backbones.
The four possible bases are adenine and guanine, cytosine and thymine.
Adenine is the complimentary base to thymine and forms two hydrogen bonds with it.
Cytosine is the complimentary base to
guanine and forms three hydrogen bonds
with it.
Base pairing allows a DNA molecule to
copy itself before cell division ­ a process
called replication.
The two strands of DNA become paired off
with each other to form a double helix
(proposed by Francis Crick and James
Watson in 1953).
NOTE: a nucleotide contains one sugar, one phosphate and one base joined together.
Crucial to the double helix arrangement are the fact that the complimentary nitrogen bases
can hydrogen bond with each other:
The interactions between other groups hold the DNA molecule into a double helix.
Protein synthesis:
It is the nitrogen bases on the DNA molecule that code for proteins.
The code in DNA is a triplet code; three bases code for each amino acid (three bases make up
each gene).
Because there are three pieces of code that are needed for each amino acid, and there are
four bases that each of the three pieces of code can take, there are 64 combinations of
code, yet there are only 20 amino acids.
Thus, some of the amino acids have more than one DNA code.

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To synthesis proteins, the information in the DNA has to get into the cytoplasm of the cell,
and then to the ribosomes.
DNA is too large to leave the nuclear envelope. A copy of the information in the gene is
made in the form of a smaller molecule; this smaller molecule is Messenger RNA, or mRNA.
mRNA is small enough to leave the nuclear envelope.
Once the mRNA strand has been synthesised from the DNA, it moves out of the nucleus to a
ribosome.…read more

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In addition to just the genes, DNA molecules also contain codes which start or stop RNA
production, and other regions which appear to have no function (exons).
The differences between RNA and DNA are:
DNA is made from the deoxyribose sugar, whereas RNA is made from the ribose sugar.
The base thymine (DNA) is replaced by Uracil in RNA.
DNA is double stranded (two sugar phosphate backbones), whereas RNA is single stranded.…read more

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The rate of reaction can be altered by changing the size of the calcium carbonate chips or the
concentration of the acid.
If the chips are large and in excess, then only a small fraction of the solid reacts and the chips
don't change shape. This means that the calcium carbonate does not affect the rate of
reaction, only the concentration of the hydrochloric acid does.
The rate will decrease over time as the hydrochloric acid is used up (its concentration
decreases).…read more

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HCl the rate will be halved, if you quarter the concentration, the rate will be
quartered.
This relationship can be converted into a rate equation by replacing the proportionality sign
with an equals sign and inserting a constant:
Rate = k[HCl]1
k is the rate constant.…read more

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Now to work out the order of the reaction with respect to X, in this case we need to use
reaction 1 and 2:
The order of this reaction with respect to Y is one, as the relationship between [Y] and rate is
directionally proportional.
The rate equation for the reaction is:
Rate = k[X]2[Y]
The overall order of the reaction is three (the two orders added together).
Now to work out the constant.…read more

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Looking at the graph, the reaction is half completed when the concentration produced is 120
cm3, at this point the concentration of HCl is 1 mol dm-3. The time taken to reach this point is
the half-life of the reaction with a starting concentration of 2 mol dm-3. The half-life for this is
141 seconds.
The time taken for the volume of CO2 to change from 120 cm3 to 180 cm3 corresponds to a
change in concentration of HCl from 1 mol dm-3 to 0.…read more

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The reason for this is that the reaction does not occur in one step, but it occurs in two steps:
The C-Br bond breaks heterolytically:
This only involves C(CH3)3Br, thus its rate is only dependent on [C(CH3)3Br].
The carbocation reacts with OH-:
The reaction for step 2 is much faster than the reaction in step 1.…read more

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If a gas is produced you can collect it in a gas syringe, the volume produced can be used to
follow the rate of a reaction. You could also measure by recording the loss in mass.
You can follow the pH of a reaction if the acid/alkali concentration changes throughout.
A colorimeter can be used to measure the change in colour of a reaction.
All these techniques do not interfere with the progress of the reaction.…read more

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The mirror images of glycine can be superimposed onto each other; the molecule just has to
be flipped vertically and the isomer is made.
D/L enantiomers
Molecules that show this kind of isomerism are called enantiomers.
Any central carbon atom that is surrounded by four different atoms/groups will form two
enantiomers.
The arrangement of atoms like this is described as a chiral centre (our hands and feet are also
examples of chiral centres, which is where the word chiral comes from- Greek for
"handedness").…read more

Comments

sabrina hasan

will you be making f335 notes on all the modules?

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