Standard Level Transcription

  • DNA double helix unzips as DNA Helicase breaks the hydrogen bonds between complementary bases seperating the two strands
  • One strand is the anti-sense and acts as a template, free RNA nucleotides are added to the exposed bases on this strand using the rules of complementary base pairing 
  • RNA Polymerase forms sugar-phosphate bonds between nucleotides
  • The mRNA detaches 
  • The two DNA strands join together by complementary base pairing 
  • The DNA molecules wind back up into a helix 

DNA provides the instructions in the form of a long sequence of nucleotides

Transcription: A complementary strand of this sequence is made in the form of pre - mRNA

The pre - mRNA is spliced to form mRNA

Translation: The mRNA is used as a template to which complementary tRNA molecules attach and the amino acids they carry are linked to form a polypeptide 

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Non - coding DNA

U5: Gene expression is regulated by proteins that bind to specific base sequences in DNA

A1: The promoter as an example of non - coding DNA with a function

What is non - coding DNA?

  • Promoter
  • Silencer
  • Enchancer

Promoter: Sequences that are attachment points for RNA Polymerases adjacent to the gene

Enhancer: Sequences that increase rate of transcription (when protein is bound to it)

Silencer: Sequences that decrease rate of transcription (when protein is bound to it)

Example of regulation of gene expression by proteins is the metabolism of lactose in the E. Coli bacterium

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Types of Regulating Proteins and Sequences

DNA Seqence: Enchancers

Binding Protein: Activator

Function: Activator proteins bind to Enhancer sequences of DNA to greatly increase the rate of transcription of a gene

DNA Sequences: Silencers

Binding Protein: Repressor

Function: Repressor proteins bind to non - coding regions of DNA to either block or reduce the transcription of a gene

DNA Sequences: Promoter

Binding Protein: RNA Polymerase 

Function: A region of DNA located close to a specific gene. Once bound to the sequence RNA transcribes the gene

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Gene Expression

U6 : The evironment of a cell of an organism has an impact on gene expression

The evironment of an organism impacts gene expression. Human hair and skin colour are impacted by the exposure to sunlight and high temperatures

Pigments in the fur of animals are also regulated by temperature 

The environment of a cell can also impact gene expression

Small number of genes are involved in determining body patterns during embryonic development

Morphogens: Expression of genes in a cell is regulated by a group of molecules

Regulate the production of transcription factors in a cell 

Diffuse arcoss the surfaces of cells from a concentrated source

Different embryonic cells get different concentrations of morphogens 

This results in the activation and inhibitation of different genes in different cells. [where nose is on face ect]

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U2 : Nucleosomes help regulate transcription in Eukaryotes 

  • DNA supercoils around 8 Histones 
  • To properly package and protect it 

Methylation: The addition of Methyl groups to DNA

Acetylation: The addition of Acetyl group to Histones

Methylation of DNA inhibits transcription

DNA binds more tightly to the Histone making it less acessible to transcription factors 

Acetylation promotes transcription

DNA binds more loosely to the Histone making it more accesible to transcription factors 

*Methylation of Histones can also occur: Both promote and inhibit transcription

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Epigenetics: Heritible changes in gene expression [no change to DNA base sequence]

Small chemical markers attach to DNA to regulate pattern of gene expression

These markers are usually passed down to daughter cells 

Example: Variation in pattern of mythelation can affect flowering time and has been inherited over generations

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DNA Methylation Patterns

S1: Analysis of changes in the DNA methylation patterns 

Hypermethylation: High levels of methylation

Similar levels of methylation

Hypomethylation: Low levels of methylation

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U1 : Transcription occurs in a 5' to 3' direction

RNA Polymerase adds the 5' end of the free RNA nucleotide to the 3' end of the growing mRNA molecule

Codons: The sequences of three bases on the mRNA coding for amino acids

Introns: Not all bases in DNA sequence code for amino acids so the mRNA just transcribed contains non - coding regions

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Splicing of mRNA

U4: Splicing of mRNA increases the number of different proteins an organism can produce

Multiple proteins are produced by a single gene. Each protein produced will vary in its biological function

Example: IgM gene which produces different immunoglobins (antibodies) to fight different pathogens

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Detailed Summary of Transcription

RNA Polymerase binds to a site on the DNA at the start of a gene

RNA Polymerase separates the DNA strands and synthesizes a complementary RNA copy from the antisense DNA strand

Transcription occurs in a 5' to 3' direction: RNA Polymerase adds the 5' end of the free nucleotide to the 3' end of the growing mRNA molecule

Covalently bonds ribonucloside triphosphates that align opposite their exposed complementary partner [uses energy from the clevage of the other phosphate groups to join them together]

Once RNA sequence has been synthesized:

  • RNA Polymerase will detach from DNA
  • RNA detaches from DNA
  • Double helix reforms

Transcription occurs in the nucleus then mRNA moves to the cytoplasm where Translation can occur

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