DNA and Protein Synthesis

  • Created by: ava.scott
  • Created on: 22-06-14 12:50

DNA functions

Allows cell division

Carries template for all protein synthesis, including for future generations

Replication allows accurate copying of DNA for cell division.

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DNA Replication: Semi-conservative theory

  • DNA helicase enzyme attaches to the beginning of the DNA. This breaks hydrogen bonds between base pairs , causing the double helix to unzip and expose bases.
  • Free nucleotides line up with complementary bases, as each chain of DNA acts as a template. DNA polymerase catalyses the reaction to form new hydrogen bonds and phospho-diester bonds between nucleotides.
  • This makes two new dna double helixes, with one old strand and one new strand.

This was proved by Meselsohn-Stahls experiment.

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Meselsohn and Stahl data


DNA from bacteria grown in 15N minerals for many generations, is extracted and is centrifuged, giving a large band at the bottom of the test tube. (Bacteria grown in 14N nutrients gave a band at the top.)

The 15N bacteria is then grown for one generation in 14N nurtients. DNA is extracted and centrifuged, giving one band in the middle.

Then the process is repeated, giving two bands, on at the middle, then one at the top. And again, and the band at the top is 3x more dense than the one at the middle.

Goes: 1 band at bottom, 1 band in middle, 2 bands (one top and one middle) 2 bands (big band at top smaller band in middle.)


This justifies semi-conservative replication, as the second generation (1 band in middle) shows that all new strands contain equal amounts of light (new) 14N DNA and heavy (old) 15N DNA.

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Protein Synthesis: Transcription

  • DNA is the starting place for protein synthesis, as the base sequence determines the primary sequence in a polypeptide.
  • Each amino acid is coded for by three bases, a triplet code (in DNA) , called a codon(IN MRNA) .
  • The portion of DNA which codes for a whole polypeptide is a gene.

Transcription is the mechanism by which the base sequence of DNA is tranferred into mRNA.

  • DNA helicase attaches at the cistron (beginning of the gene.) This breaks the hydrogen bonds between base pairs, and part of the helix unravels.
  • RNA free nucleotides (from the nucleoplasm) line up next to the exposed base pairs, and RNA polymerase catalyses the reaction to make phosphodiester bonds along the sugar phosphates of these nucleotides. This has made a strand of mRNA (DNA joins together behind RNA polymerase enzyme.)
  • Guanine is joined to exposed cytosine, and uracil to exposed adenine.
  • Only one strand of DNA is used as a template.
  • mRNA is detached, and DNA rewinds.
  • mRNA moves through nuclear pores and into cytoplasm.

Thymine is replaced by Uracil.

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Protein Synthesis: Translation

Translation by ribosomes allows the assembly of a polypeptide from the original DNA code. It is carried out by ribosmomes which are made ouyt of ribosomal RNA and protein.

  • Ribosome has two tRNA binding sites; one holds the growing mRNA sequence, and the other holds the next tRNA amino acid in the sequence.
  • tRNA molecules are specific to an amino acid and attach to the exposed bases on the mRNA.
  • A ribosomal enzyme catalyses the peptide bond fomation between the tRNA amino acid, and the growing polypeptide chain. This process uses energy from ATP.
  • The amino acid is activated by ATP, and attached the the specific tRNA, which carries the amino acid at one end, and the anticodon at the other.
  • The anticodon is what attaches to the ribosome. This process occurs, until a stop codon is reached. 
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Further modification of polypeptides

The polypeptide may be further modified and a protein can contain more than one polypeptide.

These modifications can involve the Golgi Body, where the protein is processed, modified and packaged into a vesicle.

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Meiosis I: prophase and metaphase

Meiosis results in cells containing half the original number of chromosomes.

Prophase I (6)

  • Chromosomes become shorter and thicker and are seen as two chromatids.
  • Centrioles move to the poles and microtubules radiate from then forming asters, eventually beocming the spindle.
  • Differs from mitosis: chromosome pairs associate in homologous pairs called bivalents.
  • Each bivalent consists of four strands, which are two chromosomes each with two chromatids.
  • Chromatids wrap around eachother, especially attracted at places called chiasmata. The chromosome sometimes breaks and exchange genetic material. This is called CROSSING OVER.
  • Nuclear envelope disintegrates and nucleolus dissapears.

Metaphase 1 (3)

  • Homologous pairs line up on the equator.
  • Maternal and Paternal homologous pairs are distributed randomly.
  • This is called INDEPENDENT ASSORTMENT (random distribution.)
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meiosis 1: anaphase and telophase

Anaphase I

  • Spindle fibres attach the centormere of the bivalent, seperating the sister chromatids, so each pole only gets one of each homologous pair.
  • Chromosomes reach opposite poles, and nuclear membrane develops around them.

Telophase I

  • Chromosomes remain condensed, as Meiosis II follows on. Cytokinesis occurs.
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Meiosis II

Prophase II

  • New spindle develops at right angles to the old spindle.

Metaphase II

  • Chromosomes line up on the equator, with spindle fibres attached to the centromere.

Anaphase II

  • Centromeres divide, and
  • the chromatids are pulled to opposite sides.

Telophase II

  • The chromosomes lengthen and are indistinct.
  • The nuclear membrane reforms.
  • Spindle fibre dissapears.
  • Cytokinesis takes place.
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Meiosis significance

Meiosis halves the number of chromosomes in a cell, so that when fertilisation cocurs, the zygote has a diploid number of chromosomes.

However, it also introduces genetic variation into the gametes, and therefore the zygotes. These sources of variation are essential for a species to survive in varying conditions over a long time.


  • The genotypes of each different parental gamete is mixed during fertilisation.
  • In  Metaphase I, the homologous pairs seperate completely independent of eachother, so a random assortment of maternal and paternal chromatids are distributed in each daughter cell.
  • In prophase I, crossing over at chiasmata. This produces new combinations and the separation of linked genes. This is called recombination, and a single crossing over results in 4 haploid gametes having unique genetic combination.
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