First stage on protein synthesis- DNA can't leave nucleus as too large but proteins are built on ribosomes
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so?
Genetic code copied in nucleus and transferred to ribosomes
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Transcription=
Process of copying genetic code by making mRNA from DNA
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Step 1
Start of each gene on DNA is marked by a special sequence of bases called the promoter- DNA helices breaks H bonds between base pairs
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Step 2
RNA molecule is built up from the 4 ribose nucleotides ACGU in nucleoplasm- free ribose nucleotides attach themselves to the bases on the DNA by complementary base pairing (just like DNA replication)
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What is made? what is the DNA strand copied called? What is other strand? What do free nucleotides bind to?
1 strand of RNA. Template strand or antisense strand. complementary copy called non-template or sense. Template strand
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step 3?
RNA polymerase binds to the promoter region- new nucleotides are joined to each other by strong covalent phosphodiester bonds which is catalysed by enzyme RNA polymerase
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Step 4?
Only around 8 base pairs remain attached at a time since the mRNA molecules peels off from the DNA as it is made. Winding enzyme rewinds DNA- zips up
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Step 5
At end of gene the transcription stops so the mRNA molecule is just the length of the gene
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What is post-transcription modification? why?
Removing non-coding sequences within genes- introns . As would interfere with synthesis of polypeptide before mRNA can be translated into proteins
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What are the coding sequences that code for proteins ?
Exons
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What is the process of joining functional exons?
Splicing- introns cut out and exons joined together by enzymes- some splicing done by RNA intron itself acting as RNA enzyme where are other splicing done by RNA/protein- complexes called Snurps
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what does this produce?
Shorter mature RNA containing only exons as introns are broken down
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What is the difference in Prokaryotic DNA
No introns so no mRNA processing needed
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How does mRNA move out of the nucleus ? why doesn't it have to go far to find a ribosome?
By nuclear pores into cytoplasm and binds to ribosome . As many attached to rough ER- continuous with nuclear envelope
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Then what?
tRNA brings specific AA- anticodons complementary to codon brings AA together then peptide bonds making poly peptide- primary structure (Translation)
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so?
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Genetic code copied in nucleus and transferred to ribosomes
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