DNA - Structure and Chromosomes

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DNA Structure

(http://dna02.wikispaces.com/file/view/i_am_a_whore.jpg/229173532/i_am_a_whore.jpg)

This is a NUCLEOTIDE. These are combined in a CONDENSATION reaction to make a MONONUCLEOTIDE.  More added together forms a chain, called a POLYNUCLEOTIDE. The polynucleotide chains are ANTIPARALLEL with the phosphate and deoxyribose molecules alternate to form the uprights

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The Bases

The Bases are ORGANIC, contain NITROGEN with 4 different ones in 2 groups:

SINGLE-RING BASE - CYTOSINE (C) and THYMINE (T) - these have SHORTER MOLECULES

DOUBLE-RING BASE - ADENINE (A) and GUANINE (G) - these have LONGER MOLECULES

The rungs on the DNA ladder must be the same length, with them having one base of each type

ADENINE (A) always pairs up with THYMINE (T) with THREE HYDROGEN BONDS

GUANINE (G) always pairs up with CYTOSINE (C) with TWO HYDROGEN BONDS

They are said to be complimentary to one another 

Way of remembering: George Clooney and A


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The Double Helix and Function of DNA

The double helix is made by the two polynucleotide strands being TWISTED. The uprights of the phosphate and deoxyribose wind around one another to form the DOUBLE HELIX.

This forms the STRUCTURAL BACKBONE of the DNA molecule

For each complete turn of the double helix, there is 10 base pairings. There is around 3.2 billion base parings in the DNA of a typical mammalian cell. Infinate number of base pairings provide the genetic diversity within living organisms.

FUNCTION OF DNA:

It is a HEREDITARY matieral repsonsible for passing down the genetic info from cell to cell, generation to generation. 

  • Very stable and can pass from generation to generation without change
  • Its separate strands joined together only with HYDROGEN BONDS, allowing them to SEPARATE during DNA REPLICATION and PROTIEN SYNTHESIS
  • Extremely LARGE molecule, carries alot of genetic information
  • Having the base pairs within the helical cylinder, the genetic info is mostly protected from being corrupt by outside chemicals and physical forces
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The Triplet Code

WHAT ARE GENES?:

Genes determine the nature and development of an organism. They are a sequence of DNA bases that determine a polypeptide and a polypeptide is a sequence of amino acids.

THE TRIPLET CODE:

  • Only 20 amino acids regularly occur in proteins
  • Each amino acid must have its own code of bases of the DNA
  • Only 4 bases in DNA
  • 1 base = 4 amino acids, 2 bases = 16 amino acids, 3 bases = 64 amino acids
  • So codes for amino acids must be THREE bases

FEATURES:

  • A few amino acids have 1 code, most have 2 -> 6 different codes to make same amino acid -> DEGENERATE. Also UNIVERSAL -> same for all organisms except a few
  • Read the same way in one particular direction on the DNA molecule
  • Start of sequence always the same code - methionine, removed if not part of polypeptide
  • THREE CODES dont code for any amino acid -> STOP CODES
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DNA and Chromosomes

(http://activity.ntsec.gov.tw/lifeworld/english/content/images/en_gene_c5.jpg)

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Alleles

Alleles are DIFFERENT VARIATIONS of the SAME GENE.

A gene normally exsits in 2 or more forms. 

Each individual inherits one allele from each parent. These alleles may be the same or different.

  • Each allele will code for a different polypeptide
  • If there is a difference in the sequence of an allele of a single gene --> different sequence of amino acids being coded for
  • A different polypeptide will be produced --> different protein
  • A different protein may not function properly or at all
  • Eg. An enzyme is a protein. If different amino acid sequence --> different shape which maynot allow the substrate to fit into the active site, so may not function properly
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Homologous Chromosomes

In each species, the chromosomes are always an even pair. That's because chromosomes occur in HOMOLOGOUS PAIRS, 1 from the mother, 1 from the father.

The total number of chromosomes is called the DIPLOID number. The number from 1 parent is called the HAPLOID number.

The chromsomes in a homologous pair are NOT IDENTICAL as they are 1 from each parent. They have the information for the same genes, eg.for hair colour, BUT DIFFERENT ALLELES, different codes for the same gene, eg.maternal - brown hair, paternal - blond hair.

During MEIOSIS, the halving of these chromosomes is done in a way so that each daughter cell recieves 1 chromosome from each homologous pair, so 1 set of information for each characteristic. 

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