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  • DNA is a molecule made up of two strands

  • twisted around each other in a double helix shape.

  • Each strand is made up a sequence of four chemical bases

  • represented by the letters A, C, G and T.

  • The two strands are complimentary.

  • This means that wherever there's a T in one strand

  • there will be in an A in the opposite strand,

  • and wherever there's a C

  • there will be a G in the other strand.

  • Each strand has a 5' end and a 3' end.

  • The two strands run in opposite directions.

  • This determines how each strand of DNA is replicated.

  • The first step in DNA replication is to separate the two strands.

  • This unzipping is done by an enzyme called helicase

  • and results in the formation of a replication fork.

  • The separated strands each provide a template for creating a new strand of DNA.

  • An enzyme called primase starts the process.

  • This enzyme makes a small piece of RNA

  • called a primer.

  • This marks the starting point for the construction of the new strand of DNA.

  • An enzyme called DNA polymerase binds to the primer

  • and will make the new strand of DNA.

  • DNA polymerase can only add DNA bases in one direction,

  • from the 5' end to the 3' end.

  • One of the new strands of DNA, the leading strand,

  • is made continuously,

  • the DNA polymerase adding bases one by one in the 5' to 3' direction.

  • The other strand, the lagging strand, cannot be made in this continuous way

  • because it runs in the opposite direction

  • the DNA polymerase can therefore only make this strand in a series of small chunks

  • called Okazaki fragments.

  • Each fragment is started with an RNA primer.

  • DNA polymerase then adds a short row of DNA bases

  • in the 5' to 3' direction.

  • The next primer is then added further down the lagging strand.

  • Another Okazaki fragment is then made and the process is repeated again.

  • Once the new DNA has been made

  • the enzyme exonuclease removes all the RNA primers from both strands of DNA.

  • Another DNA polymerase enzyme then fills in the gaps that are left behind

  • with DNA.

  • Finally the enzyme DNA ligase seals up the fragments of DNA

  • in both strands to form a continuous double strand.

  • DNA replication is described as semi- conservative

  • because each DNA molecule is made up of one old, conserved strand of DNA

  • and one new one.

DNA is a molecule made up of two strands

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B2 UK dna strand polymerase enzyme primer replication

DNA replication - 3D

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    申欣瑩 posted on 2016/11/08
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