Edexcel AS Biology


Structure, Genetic code, Replication, Protein Synthesis, Transcription and Translation

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Structure of DNA part 1

DNA and RNA are polynucleotides formed from subunits called nucleotides.

A nucleotide contains a:

  • Phosphate,
  • A nitrogenous base (Adenine, Thymine, Guanine, Cytosine and in RNA Uracil instead of Thymine)
  • A pentose sugar (ribose in RNA deoxyribose in DNA)

These component are joined together by condensation reactions, with the loss of water.

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Structure of DNA part 2

Two seperate strands are held together by hydrogen bonds. They wind around each other in a double helix.

DNA forms complementary base pairing as Adenine always pairs wirh Thymine and Guanine always pairs with Cytosine.

Adenine and Guanine are large with 2 rings they are the purines and Cytosine and Thymine have 1 rings. One large pairs with smaller base so width of a base pair is always the same.

Hydrogen bonding is due to the shape of the molecules two Hydrogen bonds form between Adenine and Thymine whereas three form between Cytosine and Guanine.

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Genetic Code

The order of bases on one strand of DNA is the genetic code.

The code is formed from triplets of bases, each triplet codes for an amino acid.

The sequence of triplets codes for the sequence of amino acids that will form a polypeptide, which will fold up to form a protein.

A gene is a sequence of bases on one strand of a DNA double helix molecule which codes for a polypeptide chain.

The genetic code must:

  • Self replicate so that copies can pass to duaghter cells during cell division
  • Carry information that codes for proteins.
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During replication:

  • The two strands of DNA unwind and split apart.
  • The free nucleotides line up along each strand, as the complementary base pairing.
  • The enzyme DNA polymerase bonds the nucleotides together as a phosphodiester bond forms between each deoxyribose and phosphate group.
  • Hydrogen links the two strands together.
  • Two identical daughter strands are created.
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Protein Synthesis

The mRNA passes into the cytoplasm through nuclear pores and is used to make a polypeptide in a process called translation.

The gentic code is in the nucleus, but the protiens formed using the code are made in the cytoplasm.

DNA code is copied making a molecule of messenger RNA in a process called transcription.

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During Transciption:

  • The ribonucleotides are joined by RNA polymerase to form a strnad of mRNA.
  • Ribonucleotides are paired with their complement on the template strand: uracil pairs with adenine instead of thymine.
  • Only one strand is used in the formation of mRNA- the template (antisense) strand.The unused strand is called the sense strand.
  • During transcription the DNA unwinds and hydrogen bonds between base pair split to seperate the two strands.
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During Translation:

  • The mRNA carries the gentic message in the same base sequence language as the DNA.
  • Transfer RNA (tRNA) translates the base sequence on th mRNA into the protein amino acid sequence.
  • Each tRNA molecule carries an amino acid to the mRNA, where the amino acid joins others carried by other tRNA to build a polypeptide.
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