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Transcription: Gene expression.
Transcription: Gene expression.
This section includes the IB topic 2.7 and 7.2:
This section includes the IB topic 2.7 and 7.2:
- Transcription is DNA expression: synthesis of mRNA from a gene (in a 5’ to 3’ direction)
- Co-transcriptional modification of RNA: from pre-mRNA to mature mRNA (splicing)
- mRNA export: from the nucleus to the cytoplasm - Dr. Pena's Ph.D. (linked below for curious people)
- Effects of the environment on gene expression.
- DNA methylation and histone acetylation
- Retro-transcription: synthesis of cDNA from RNA (by retrovirus and in biotechnology)
Dr. Pena's whiteboard
Dr. Pena's whiteboard
Dr. Pena's summary:
Transcription is the synthesis or mRNA from a DNA template (called the antisense strand). mRNA is synthesized as the antisense strand is transcribed.
The enzyme RNA polymerase, together with a group of proteins called transcription factors, binds to the promoter region of DNA (antisense strand) at the beginning of the gene. RNA polymerase has helicase activity and separates the two DNA strands, unwinding the double helix and exposing between 10 and 20 DNA bases at a time, for pairing with RNA nucleotides.
RNA polymerase together with the transcription factors travels along the gene (towards the 5' of the antisense strand). RNA nucleotides are added by complementary base pairing to match the sequence of the DNA antisense strand. RNA polymerase pairs DNA nucleotides with RNA nucleotides. In RNA, adenine pairs uracil and guanine with cytosine. Hydrogen bonds are formed between RNA nucleotides and DNA nucleotides in the antisense strand. As RNA polymerase moves along the gene, the DNA double helix is unzipped and zipped back.
The RNA polymerases uses RNA nucleoside triphosphate in transcription. The hydrolysis of two phosphates molecules provides energy for the formation of phosphodiester bonds between RNA nucleotides in the growing mRNA strand.
RNA polymerase adds RNA nucleotides to the 3' end of the growing mRNA molecule. The mRNA is synthesized in a 5' to 3' direction. The RNA polymerase moves along the DNA antisense strand (template) in a 3' to 5' direction (antiparallel). The sequence of mRNA matches the sense strand of DNA (5' to 3').
A terminator sequence in the antisense strand signals the end of the gene (end of transcription). The RNA polymerase and the mRNA molecule separate completely from the DNA double helix. In eukaryotic cells the mRNA undergoes splicing and addition of a polyA tail at the 3' end and a 5' cap at the 5' end. In prokaryotes, mRNA is translated as it is synthesized: transcription is coupled to translation (no nucleus, DNA located in the cytoplasm, together with ribosomes).
RNA processing: in eukaryotic cells, introns in the mRNA are removed in a process called splicing. A set of proteins is added to the 5' end (called the 5' cap) and a sequence of Adenines is added at the 3' end (called the poly-A).
RNA export: in eukaryotic cells, mRNA is compacted with proteins (mRNP) and exported to the cytoplasm where it reaches a ribosome (free standing or polyribosome) and is used as a template to synthesize a polypeptide (in a process called transcription).
Transcription and gene expression.
Transcription and gene expression.
transcription-direction.mpeg.mp4RNA splicing:
RNA splicing:
Humans have about 25.000 genes. We don't know how many different polypeptides we have, but the number is far greater than the number of genes. How is that posible? The fascinating discovery of alternative splicing, a mechanism by which the cell (through the spliceosome complex) can produce multiple polypeptides from the same gene.
These slides summarize and illustrate the important concepts of transcription 7.2. The worksheets aside, contains some practice questions on "transcription factors" and some IB-Style data questions on "regulation of gene expression by methylation" (diagnosis of lung cancer) - from InThinking.
Transcription.pdfTranscription_factors.pdfTiny answers:
1 e.g. genes for processes like glycolysis. 2 Proteins which migrate into the nucleus and activate gene transcription. 3 In the nucleus4 These proteins ensure that transcription of genes in the cell occurs appropriately. They indicate the beginning of genes which are being expressed. RNA polymerase cannot bind to a promoter by itself, it requires these protein factors added to the promoter to help it bind, and then transcribe the mRNA. This gene expression depends on signalling within the cell or environmental factors.Tiny answers:
Methylation_DNA.pdfWhy media instead of average? Read this
Epigenetics: the environmental effect on gene expression.
Epigenetics: the environmental effect on gene expression.
DNA-methylation_patterns.ppt
Retrotranscription
Retrotranscription
Also called reverse transcription, is the synthesis of DNA form RNA. It is used by viruses which genome is in the form of RNA (retrovirus) or in the biotechnology industry.
RETROVIRUS - Viral infection requires the genetic material of the virus to be combined with the cellular genome. Viruses that contain genes in RNA (retroviruses) use an enzyme called reverse-transcriptase that converts the RNA into DNA. They use this enzyme to convert their RNA genes into DNA genes. DNA genes can then be inserted into the cell's genome, which is always in the form of DNA. Some anti-viral drugs, such as those used to treat HIV, inhibit the enzyme reverse transcriptase, because HIV is a retrovirus. The treatment slows down the rate of infection of the HIV virus to T-Cell lymphocytes. If you like to learn more about this, check this out!
BIOTECHNOLOGY - Most human genes contain introns. Some biotechnology techniques aim to introduce human genes in other organisms, such as the insertion of the human insulin gene in bacteria (so that bacteria produces human insulin). Because prokaryotic genes have no introns, splicing cannot take place in bacteria. The only way to produce human insulin in bacteria is isolating the mature mRNA from a human cell and making DNA (without introns) from it. That is be done using the viral retro-transcriptase enzyme. Human cells are grown and the mature mRNA from insulin is isolated. Then the mRNA is converted into DNA (called complementary DNA or cDNA) with the enzyme retro-transcriptase. Then the cDNA is inserted into a bacteria by using a plasmid. If the bacteria incorporates the plasmid into its genome, the bacteria will express the human gene for insulin and produce human insulin, that can then be collected and distributed to patients suffering from diabetes. This prevents many ethical issues, such as extracting insulin from pigs (or other mammals). If you are interested, read more about that here. We will talk more about this and other applications of biotechnology in the unit of genetics... Biotechnology is the best! :-)
Watch the videos below if you are interested in learning more about retrotranscription, it is very cool!
Vocabluary_7.2.pdf
If you are curious... find below Dr. Pena's paper, published in 2012 in the European Molecular Biology Organization journal.
If you are curious... find below Dr. Pena's paper, published in 2012 in the European Molecular Biology Organization journal.
We investigated and discovered the structure of the THO complex, a protein complex that travels along the gene with the RNA polymerase and the transcription factors. The THO complex prepares the mRNA for export by loading proteins along the mRNA strand. As a result, the mRNA assembles with multiple proteins in an mRNP complex (messenger ribonuceoprotein). We believe that the THO complex plays a role in the targeting of the mRNA to specific ribosomes in the cytoplasm.
mRNA_Export_THOcomplex_Pena2012.pdfSee the article (and many others) published here.
7.2 Transcription.pdfReport abuse