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Structure of DNA 8.1
One Nucleotide
- Deoxyribose/ Ribose and organic base bind through a condensation reaction to form One Nucleotide.
- Two Nucleotides bind to form Dinucleotides through condensation reactions.
- Dinucleotides bind to form polynucleotides through Condensation reactions.
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Structure of DNA 8.1
DNA Structure
- * Thymine is Uracil in RNA.
- C and G form 3 hydrogen bonds.
- A and T form 2 hydrogen bonds.
- C And G are Complimentary.
- A and T are Complimentary.
- DNA forms a double helix.
- Each turn in the double helix has ten base
Advantages for the Functions of DNA:
- It is stable and can pass down generations
without changing.
- It is joined by hydrogen bonds, which are
weak and break easily for DNA replication
and protein synthesis.
- It is extremely large so can carry a lot of
- Deoxyribose-phosphate backbone protects
the genetic information.
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The triplet code 8.2
The triplet code
- Genes are sections of DNA on a chromosome coding for one or more polypeptides.
The Triplet code:
- Only 20 amino acids regularly occur in proteins.
- Each amino acid must have its own code of bases on the DNA.
- Only four different bases are present in DNA.
- If each base coded for a different amino acid, only four amino acids could be coded for.
- Using a pair of bases, 16 (42) different codes are possible, which is still inadequate.
- Three bases produce 64 (43) different codes, more than enough to satisfy the requirement
of 20 amino acids.
- So the code has Three Bases called the triplet code.
- In eukaryotic cells, DNA has parts that do not code for amino acids which are called
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Slide 4

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DNA and chromosomes 8.3
Chromosome Structure
- In Prokaryotic cells,
the DNA molecules
are smaller and form
a circle. They do not
associate with
protein molecules
and so do not have
- In Eukaryotic cells,
the DNA molecules
are larger and form a
line. They associate
with proteins to
form structures
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Slide 5

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DNA and chromosomes 8.3
Homologous chromosomes and alleles
- The Sperm cell contains the paternal chromosomes and the egg cell contains the
maternal chromosomes and these chromosomes are called the homologous
- The total number of chromosomes are called the diploid number.
- Determining the same genetic characteristics is not the same as being identical.
- A homologous pair is always two chromosomes that determine the same genetic
- Each gene exists in pairs in two or more different forms. These forms are called
- An individual inherits one allele from each of its parents.
- Alleles code for a different polypeptide.
- Changes in the base sequence of an allele may result in a different sequence of
amino acids which can cause the proteins to not function properly.
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Haemoglobin 10.1
- The role of haemoglobin is to transport oxygen.
- To transport oxygen efficiently:
- Readily associate with oxygen at the surface where gas exchange takes place,
- Readily dissociate from oxygen at those tissues requiring it.
Oxygen carbon dioxide affinity of haemoglobin for
Region of body concentration concentration oxygen result
Gas exchange oxygen
surface high low high association
Respiring tissues low high low dissociation
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