本期的內容是轉錄。本文集的這一部分是遺傳、進化與生態(tài)學 Genetics, Evolution, and Ecology. 這門課理論上建議在閱讀完文集的第一部分的內容之后再開始學習，但基礎不足的朋友也可以嘗試閱讀喔~

這一部分的主要內容均來自 Prof. Angela J. Roles 的 BIOL 200 課程，因此本文集的這一部分均不會標記為原創(chuàng)。但由于文本來源不清晰，UP主還是一個字一個字碼出來的文章，本文禁止非授權的轉載，謝謝！

Lesson 9: Transcription

[1] Overview

- How do we get from genotype to phenotype?

First, cells must produce proteins. That requires that genes are transcribed into RNA which is then processed and translated into protein.

?Transcription = creation of RNA from DNA;

?RNA splicing = processing the RNA to remove regions that will not be included in the final protein;

?Export = preparing the processed RNA to be moved into the cytosol and translated into protein;

?Translation = building of a protein according to the sequence of the messenger RNA;

?Transcription = creation of RNA from DNA;

?Consider a eukaryotic cell in interphase, performing usual cellular functions;

????- Chromosomes not super-coiled, nucleosomes still present;

?Genes are being expressed to produce proteins via transcription and then translation.

?

Overview of transcription

?Cells regulate which proteins are synthesized, when, and for how long in response to signals in their environments (internal and external).

?Constitutive genes?are expressed by a cell nearly all the time at a relatively constant level (sometimes called housekeeping genes)

????- Different cells types may have different sets of constitutive genes (e.g., brain cells versus kidney cells).

?Most genes are facultative; they are only transcribed when needed and in response to stimuli.

????- Regulatory molecules respond to stimuli to start, stop, or adjust transcription.

?Cell receives signals from environment → transcription factors?are activated/de-activated and/or epigenetic signals are added to (or removed from) DNA in response.

?

[2] Example: Temperature-dependent sex determination in turtles

Masumoto and Crews (2012) studied environmental sex determination in the red-eared slider (Trachemys scripta)

?Temperature fertilized eggs experience in the nest directs gonad type;

????- Males result from incubation at 26℃ (MPT, male-producing temperature);

????- Females result from incubation at 31℃ (FPT, female-producing temperature);

????- 29.2℃ yields 50/50 sex ratio.

?

Gene expression varies depending on incubation temperature

?Masumoto and Crews (2012) found that expression (amount of mRNA produced) varies with temperature for these transcription factor genes (FoxL2 and Dmrt1)

FPT = female-producing temperature; MPT = male-producing temperature

What happens if...

What do you think would happen if a turtle had FoxL2 alleles that...

?????Couldn’t produce any mRNA or?produced a protein that could not do?its job?

?????Made functional protein but was?transcribed at the wrong time, too?late, for example?

?????Encoded functional protein BUT the?turtle made way more mRNA than?usual?

THE POINT: there are many ways for gene expression to be different from?“normal”. Some of them cause problems and others do not or simply cause?individuals to vary slightly from the average.

[3] Transcription Initiation

What determines whether or not a gene is transcribed?

(1) Cell receives signals from environment

????- For example: the temperature changes, causing some proteins to change?shape (altering their ability to function).

????-?For example: a new food source is available and detected by cell surface?proteins.

(2) As a result, transcription factors are activated/de-activated and/or epigenetic signals are added to (or removed from) DNA in response

????- For example: transcription factors bind to DNA upstream of a gene and attract the attention of the transcription machinery.

????- For example: a methyl group is added to the DNA upstream of a gene and thus a transcription factor binding site is blocked, impeding recruitment of the transcription machinery.

?

Once signals are detected by the cell...

Transcription factors bind to the DNA upstream of a gene (promoter and enhancer sequences), “attracting” RNA polymerase to the site.

?????RNA pol (and friends) scan dsDNA in the nucleus, find and bind to now-available promoter region, initiating transcription.

?????Single-stranded messenger RNAs (mRNA) are transcribed from the template DNA strand (by RNA pol) and then processed into mature mRNA.

?

[4] Regulation

How can a cell increase or decrease production of a given protein?

STOP and pause to do the following exercises:

????(1) Describe 2 ways a cell could increase protein production.

????(2) Describe 2 ways a cell could decrease protein production.

Note: A good response includes more than saying “chromatin remodeling” or “translational control.”

?

Example good responses

?Describe 2 ways a cell could increase protein production

????- The cell could make more mRNA (transcriptional control)；

????-?A?cell could produce proteins that improve RNA stability and reduce the?rate of degradation；

????-?A?cell could remodel the chromatin so that the gene to be transcribed is?more physically available (less coiled, less methylated).

?Describe 2 ways a cell could decrease protein production

????- A?cell could mark the mRNA to be destroyed before it could be translated;

????- A?cell could block the promoter of the gene so transcription can’t occur;

????- (In many ways these responses are the opposite of the first prompt!).

[5] Regulatory Sequences

Regulatory sequences: Enhancers increase transcription rate

Regulatory sequences alter the rate of transcription.

?????An enhancer sequence increases transcription when an activator protein (a transcription factor) binds to the enhancer.

?????Regulatory sequences can be distant from the gene but may interact with other proteins to bring them closer to the promoter.

?Regulatory sequences: Silencers decrease transcription rate

????A silencer sequence (or operator) reduces transcription when an active repressor protein (a transcription factor) is bound to the silencer.

Enhancers and silencers can be up to 50,000 bases away from the gene’s promoter.

[6] Transcription Complex

The eukaryotic transcription complex

?????Transcription involves the coordinated activity of many different proteins;

?????Notice that for a single gene, transcription may be influenced by multiple enhancers or repressors;

?????Vocab to recognize: coding region, promoter, RNA polymerase, enhancer, silencer, activator, repressor.

????Note: the TATA box is a specific type of promoter sequence that interacts with the TATA-binding protein.

?

[7] Regulation Example

Example of regulatory sequences influencing?transcription

????Myers et al. (1986, Science) mutated each nucleotide?upstream of the β-globin?gene and then measured how much mRNA was produced.

Mutating the highlighted sequences reduces transcription rate of the β-globin?gene.