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DNA (AS Biology)

DNA structure, function and replication. [not finished]

George H

on 26 April 2013

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Transcript of DNA (AS Biology)

DNA Structure Functions of DNA The
Code Features of
the Triplet Code Arrangement of bases codes for arrangement of amino acids... Experiment:
DNA Extraction from
Kiwifruit! Double helix (10 base pairs for each complete turn
a Polynucleotide- deoxyribonucleic acid
Made up of two polynucleotide strands held together by hyrogen bonds between bases. Carries an immense amount amount of genetic information! Phosphodiester
Bond Carbon-Pentose Glycosidic
Bond A Single nucleotide Diagram The two strands are known as the 5' (5 Prime) & 3' (3 Prime).

This name is used as it is the number of the carbon in the pentose which attaches to the phosphate group in the 1st nucleotide. nucleotide Strands are anti-parallel C = cytosine
G = guanine
A = adenine
T= thymine C & G = complementary bases A & T = complementary bases Why? A and G are both purines with a double ring structure. C and T are pyrimidines with a single ring structure. This pairing ensures that the paired bases
are always the same length! "...two polynucleotide strands held together by hydrogen bonds between bases...." [T] thymine [A] adenine

[C] cytosine [G] guanine pyrimidines purines It is extremely large (but highly condensed to reduce space usage) 'super coiled' Must resist corruption by outside
physical and chemical forces! So base pairs are within the helical cylinder of the deoxyribose-phosphate backbone Because they must pass from generation
to generation without change! Very stable Must separate during replication & protein synthesis ...so polynucleotide strands are only joined by easily broken hydrogen bonds Structural


Proteins physical traits

metabolic traits

control of growth
& development A minimum of 3 bases codes for an
amino acid, because... only 20 amino acids are regularly occuring in proteins

each amino acid must have its own code of bases on the DNA

only 4 bases (A,C,G,T) are present in DNA: if each base coded for 1 amino acid, only 4 different amino acids could be formed...

using 2 bases 4^2 = 16 not enough!

using 3 bases 4^3 = 64 more than enough for the 20 regularly occurring amino acids...

some amino acids have more than 1 code
much DNA does not code for amino acids (introns) A 'degenerate code' because most amino
acids have more than 1 triplet code (between 2-6) Always read in one direction
along the strand from 5' to 3' 5' 3' Non-overlapping: each base sequence is read only once!

Always starts with the code for the amino acid methionine which is removed later (if necessary) 3 triplet codes do not code for
an amino acid, but instead for
'stop codes' that mark the end
of a polypeptide chain And finally The triplet code is universal;
With a few minor exceptions it is the same
in all organisms. Changing the code can change the amino acid, resulting in different bonding and tertiary structure! Sliced kiwis are
placed in a zip lock bag.

20ml of the shampoo solution is added & the bag is closed tightly without much extra air. Shampoo Solution 1 part shampoo: 10 parts water
15g of table salt
20ml total for each zip lock bag The kiwi is then thoroughly mushed with the solution inside the bag.

This breaks the cell membranes, and pushes out the cell contents... The mixture is cooled
in an ice bath for a minute. It is mushed, then cooled again several times. The mixture is then filtered through a cheesecloth.

This separates some of the cell walls and organelles from the DNA. The cooling stops
enzymes from
digesting the DNAses The kiwi solution is added to
test tubes (approx. 3ml each) Then 2ml of cold 95% ethanol is added in addition to the kiwi solution.

This separates the DNA from the rest of the solution. The cold solution prevents any digestion of DNA. A white substance congregates at the top portion of the liquid- this is the DNA! The end. Cancer A group of diseases (around 200 in total)
Caused by damage to genes that regulate mitosis and the cell cycle
Leads to the uncontrolled growth of cells (forming a constantly expanding tumour) Treatment Usually involves blocking some part of the cell cycle and hence cell divisions. Drugs used in chemotherapy do this by:

preventing DNA from replicating (e.g. cisplatin)

inbiting the metaphase stage of mitosis by interfering with spindle formation (e.g. vinca alkaloids)

These drugs also affect the normal cells but are more damaging to cancer cells because they divide rapidly. Normal cells (e.g. hair-producing cells) that divide rapidly are also vulnerable to damage. Most likely to develop in: lungs
prostate gland
large intestine
pancreas Genetics Definitions Allele Different versions of the same gene Base A (chemically) basic part of a nucleic acid. Adenine (A), thymine (T), cytosine (C), guanine (G) and uracil (U). A always pairs with T and C with G Chromosomes Well-defined rod-like single chains of DNA as arranged before the DNA is replicated OR two chromatids chains after the single rod-like chain duplicated itself. Chromatin Can refer to the structure of the chromosome (combination of proteins + DNA) or the mess of multiple chromosomes when they are in the nucleus and not preparing to divide or diving. Chromatid One of the two chains that make up a chromosome after it has replicated itself. Once the two chromatid chains separate (are no longer connected by the centromere) they are then called chromosomes again. DNA Deoxyribose Nucleic Acid- contains two polynucleotide strands joined together with hydrogen bonds between the bases
(AT=2, CG=3) DNA
polymerase An enzyme that makes DNA from nucleotide triphosphates. Usually it requires a single stranded template (DNA or RNA) and a short primer (DNA or RNA) to enable it to make a complementary copy. Gamete Reproductive (sex) cell that fuses with another gamete during fertilisation. Gene Sections of DNA coding for one or more polypeptides. Gene Pool The total number of alleles in a particular population at a specific time. Homologous
pairs of
chromosomes Matching chromosomes (one form each parent) containing the same genes in the same order Histone The proteins that the DNA wrap around. They help give the DNA shape. Intron The non-coding regions of DNA. Exons The coding regions of DNA Locus A specific position on the DNA chain Nucleotide A building block of nucleic acid consisting of a base (adenine, thymine, cytosine, guanine, uracil) joined to a sugar (ribose or deoxyribose) and a phosphate. Purine A purine is a heterocyclic aromatic organic compound (double ringed structure).

e.g. Adenine and Guanine Pyrimidine An aromatic heterocyclic organic compound (single ringed structure).

e.g. Thymine and Cytosine The Cell Cycle
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