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Transcription and gene expression

Topic 7
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Crick in 1956
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It states that information passes from genes on the DNA to the RNA copy

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The RNA copy directs the production of proteins controlling the sequence of
amino acids

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It can be summarized as:
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DNA =transcription=> RNA =translation=> PROTEIN

DNA ⇒ RNA
➢ We only have one active enzyme in transcription called RNA polymerase
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It separates the two strands

➢ The RNA polymerase finds a promoter region which tells RNA polymerase where to
start
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Usually 5’- TATAAA -3’ => the TATA box

➢ Nucleoside triphosphates (dNTPs) are added complementary to one of the strands
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Two phosphates are released and the backbone is formed
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Formation of phosphodiester bonds

Nucleosides are added in the 5’ to 3’ direction

➢ Transcription continues until a terminator region is transcribed
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When this happens the RNA transcript (mRNA) is detached from the DNA

➢ The DNA strands are rewound
Which strand is copied?
➢ The strands are complementary, so there is a difference in the code of each
➢ Punctuation codons are the start and end codons
➢ The two strands have different sequence of amino acids = different codons =
different proteins
➢ The DNA strand that carries the genetic code is called the sense strand or coding

strand
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The other is called the antisense or template strand

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The mRNA molecule has to be a copy of the sense strand, so the antisense
strand is transcribed

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Transcription and gene expression

Topic 7
...
2

Biology HL

Regulators of transcription
➢ Nucleosomes
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If DNA is wrapped around the histones it cannot be transcribed

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A methyl group (CH3) is an organic functional group that causes a section of
DNA to wrap more tightly around histones
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Prevents transcription of that particular allele

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The gene becomes inactive, not expressed

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The methylation patterns persist through cell division = keep on
being methylated

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Example: mammalian females have two X chromosomes, one become
heavily methylated = inactive

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Methylation may regulate the expression of either the maternal or paternal
allele of a gene

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Some methylation patterns are also associated to cancer
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Hypermethylation and hypomethylation
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Both due to carcinogens and toxic substances

Can be used to diagnose cancer

Sometimes methylation goes wrong and genes that were supposed to be
active are inactive and vice versa

➢ Binding proteins
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Proteins can regulate transcription by assisting or hindering the binding of
RNA to the promoter

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Transcription activators cause looping of DNA, which results in a shorter
distance between the activator and the promoter region of the gene
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Assist the binding of RNA polymerase

The repressors bind to segments of DNA called silencers preventing
transcription of that segment

➢ The environment of the organism
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Recent science has shown that the environment of the organism influences
genes
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The science is called epigenetics

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Example: people in urban areas show expression of more respiratory
genes than people from rural areas

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Genes can be methylated or unmethylated

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