7.2 TRANSCRIPTION AND GENE EXPRESSION
- Created by: lineventer
- Created on: 10-02-20 16:41
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
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
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
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]
Nucleosomes
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
Epigenetics
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
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
Transcription
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
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
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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