Genetic information, Variation and Relationships Between Organisms

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  • Created by: Mary_07
  • Created on: 05-06-17 19:06

DNA in Prokaryotes

  • Short
  • Circular
  • Not associated with histones (proteins)
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DNA in Eukaryotes

  • Linear
  • Long 
  • Associated with histones (proteins)
  • A DNA molcule and its associated proteins form a chromosome
  • The mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotes, is short, circular and not associated with proteins.
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What is a Gene?

A gene is a base sequence of DNA that codes for:

  • a functional RNA molcule
  • an amino acid sequence of a polypeptide

Each gene has a specific position on a chromosome, called its locus.

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Non-Coding DNA

Non-coding sections of DNA are termed introns, these are found between gene in multiple repeats and are also found within genes. 

The coding sections are called exons.

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The Triplet Code

3 DNA bases code for one amino acid. 

The sequence is called a triplet.  

Example:

CGG is the code for the amino acid alanine.

Degenerate code - most amino acids are coded for by more than one triplet

Non-overlapping - each base is only read once

Universal - each triplet codes for the same amino acid in all organisms

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Protein Synthesis (Part 1)

Genome - the complete set of genes in a cell

Proteome - the full range of protein that a cell is able to produce

mRNA - straight chain, single chain, base sequence is complementary to DNA code, carries the 'blue-print' from the nucleus to the cytoplasm, contains codon (sequence of 3 bases coding for an amino acid)

tRNA - clover leaf shape, carries amino acids to the ribosomes, amino acid binding site, contains anticodon (complementary to codon on mRNA)

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Protein Synthesis (Part 2)

Transcription (takes place in the nucleus)

  • DNA helicase attaches to and unwinds the DNA double helix, at the site to be transcribed.
  • Hydrogen bonds between the 2 strands are broken and the strands separate.
  • One strand is used as a template.
  • Complementary free RNA nucleotides join with the exposed DNA nucleotides, via base pairing.
  • U = A C = G
  • RNA polymerase joins these nucleotides together.
  • mRNA formed is complementary to DNA and is called pre-mRNA as it contains introns and exons.
  • Introns are removed through the process of splicing.
  • Exons are joined to produce an RNA molecule that only contains coding regions in the gene.
  • mRNA passes out of the nucleus via nuclear pores.
  • mRNA goes to the ribosome.
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Protein Synthesis (Part 3)

Translation (in the cytoplasm)

  • mRNA attaches to the ribosome.
  • tRNA carrying a specific amino acid binds to the first codon on mRNA, via complementary base pairing. 
  • A second tRNA carrying a specific amino acid binds to the second codon on mRNA, via complememntary base pairing. 
  • Enzymes join amino acids together forming a peptide bond, using energy from the hydrolysis of ATP.
  • Ribosome moves along to the next codon.
  • The tRNA lost from the first codon, it no longer has an amino acid, which moves away to collect another of the same amino acid.
  • Process repeats, with the addition of an amino acid using energy from the hydrolysis of ATP until the stop codon is reached.
  • Polypeptide is released.
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