7.2 Transcription and gene expression Understanding: Gene expression is regulated by proteins that bind to specific base sequences in DNA The environment of a cell and of an organism has an impact on gene expression Nucleosomes help to regulate transcription in eukaryotes Transcription occurs in a 5’ to 3’ direction Eukaryotic cells modify mRNA after transcription Splicing of mRNA increases the number of different proteins an organism can produce Applications: The promoter as an example of non-coding DNA with a function Skills: Analysis of changes in DNA methylation patterns Nature of science: Looking for patterns, trends and discrepancies: there is mounting evidence that the environment can trigger heritable changes in epigenetic factors

Non coding DNA Coding Not all DNA codes for a polypeptide to be made May have another useful function Non-coding sequences of DNA Example: Promotor Non Coding Applications: The promoter as an example of non-coding DNA with a function

Promotors Located near a gene Binding site of RNA polymerase Initiates transcription Promoters are not transcribed into RNA Same sequence for most genes (general RNA polymerase binding) Applications: The promoter as an example of non-coding DNA with a function

Gene Expression Some proteins not regulated (essential for survival so are expressed all the time) Some produced at certain times, in certain amounts Different causes: Variation in environmental conditions Cellular differentiation Understanding: Gene expression is regulated by proteins that bind to specific base sequences in DNA

Different proteins used to regulate transcription Gene Expression Different proteins used to regulate transcription Different sections of DNA bases where these proteins bind Unique to the specific gene they are regulating Enhancers Silencers Promoter-proximal elements Understanding: Gene expression is regulated by proteins that bind to specific base sequences in DNA

Promoter-proximal elements Promoters Promoter-proximal elements Enhancers Silencers

Promoter proximal elements Near to promoter Specific sequence for each gene Contains other regulatory elements Understanding: Gene expression is regulated by proteins that bind to specific base sequences in DNA

Enhancers Silencers Increase the rate of transcription Does not need to be near promoter Decrease the rate of transcription Does not need to be near promoter Understanding: Gene expression is regulated by proteins that bind to specific base sequences in DNA

Promotors Located near a gene Binding site of RNA polymerase Initiates transcription Promoters are not transcribed into RNA Same sequence for most genes (general RNA polymerase binding) Applications: The promoter as an example of non-coding DNA with a function

Nature vs Nurture EYE COLOUR SCARS TATTOOS Your characteristics are due to your environment, your genes, or a bit of both. Put the characteristics on the scale ACCENT WEIGHT SUN TAN HAIR COLOUR INTELLIGENCE Environmental Inherited Understanding: The environment of a cell and of an organism has an impact on gene expression

Gene Regulation Production of skin pigmentation during exposure to sunlight Melanin produced to protect against UV rays from the sun Gene regulation in response to environment Understanding: The environment of a cell and of an organism has an impact on gene expression

Gene Expression Many studies on identical and non identical twins raised together or apart More evidence for intelligence is inherited Mr. Burford: THE JIM TWINS Understanding: The environment of a cell and of an organism has an impact on gene expression

Prior Learning DNA molecules are paired with a protein called histone Histones help to package DNA 8 histones make up a nucleosome Essential as DNA can be 4cm long, it must fit into a microscopic nucleus Understanding: Nucleosomes help to regulate transcription in eukaryotes

Impact the visible characteristics of an individual Nucleosomes Chemical modification of histones is important in whether a gene is expressed or not. Add: Acetyl group (COCH3) Methyl group (CH3) Phosphate group (PO4) Increase or decrease the accessibility of the gene to transcription factors Impact the visible characteristics of an individual Epigenetic tags Understanding: Nucleosomes help to regulate transcription in eukaryotes

Methylation Direct methylation of DNA decreases gene expression Methylation of histones can increase or decrease gene expression Skills: Analysis of changes in DNA methylation patterns

Epigenetic Tags Evidence that chemical modifications that occur to DNA in one generation may be passed onto the next Sum of epigenetic tags in an organism = epigenome Nature of science: Looking for patterns, trends and discrepancies: there is mounting evidence that the environment can trigger heritable changes in epigenetic factors

Transcription starts at a promoter Initiation (start at promoter) Elongation (build) Termination (RNA completed and breaks off at terminator) 5’ 3’ 3’ 5’ Transcription starts at a promoter 5’ to 3’ direction Understanding: Transcription occurs in a 5’ to 3’ direction

Initiation RNA polymerase binds to promoter DNA unwinds H bonds break Strands separate Adds the 5’ end of a free nucleotide to the 3’ end of the growing mRNA molecule Understanding: Transcription occurs in a 5’ to 3’ direction

Elongation Enhancers speed up rate of transcription Silencers slow down rate of transcription Free RNA nucleotides attracted to complementary base pairs (AU, CG) Nucleotides joined by RNA polymerase in 5’ to 3’ direction Understanding: Transcription occurs in a 5’ to 3’ direction

Termination RNA reaches terminator and detaches H bonds break RNA molecule detaches from DNA DNA forms double helix again (H bonds form) Many RNA polymerases can follow one another Understanding: Transcription occurs in a 5’ to 3’ direction

Intiation Elongation Termination

Post Transcriptional Modification Prokaryotes: No nuclear membrane: transcription and translation coupled Eukaryotes: can carry out modification before mRNA leaves nucleus Understanding: Eukaryotic cells modify mRNA after transcription

Post Transcriptional Modification Pre-mRNA  mature mRNA RNA splicing (Introns removed and exons joined together) Understanding: Eukaryotic cells modify mRNA after transcription

Post Transcriptional Modification Allows one gene to code for multiple proteins Must have multiple exons Exons may or may not be included in mRNA Understanding: Splicing of mRNA increases the number of different proteins an organism can produce

Exam Question Explain the process of DNA transcription leading to the formation of mRNA (8 marks)

Mark Scheme RNA polymerase; (polymerase number is not required) binds to a promoter on the DNA; unwinding the DNA strands; binding nucleotides to the DNA; it moves along in a 5′→3′ direction; using complementary pairing/Adenine-Uracil and Cytosine-Guanine; until a terminator signal is reached; RNA detaches from the template and DNA rewinds; RNA polymerase detaches from the DNA; many RNA polymerases can follow each other; introns have to be removed in eukaryotes to form mature mRNA; 8 max