Topic 7.2: Transcription and Gene Expression Flashcards Preview

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Flashcards in Topic 7.2: Transcription and Gene Expression Deck (27)
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1

Gene

Sequence of DNA which is transcribed into RNA

2

Sections of a Gene

1) Promoter (transcription initiation site)
2) Coding sequence (the region transcribed)
3) Terminator (transcription termination site)

3

DNA Double strand and transcription

1) The antisense strand is transcribed into RNA (complementary)
2) The sense strand is not transcribed into RNA
(identical)

4

Transcription

Process by which a DNA sequence is copied into a complementary RNA sequence by RNA polymerase

5

Steps in transcription

1) Initiation
2) Elongation
3) Termination

6

Initiation

1) RNA polymerase binds to a promoter and unwinds DNA
2) It breaks the H bonds between complementary bases
3) Nucleoside triphosphates bind to complementary bases

7

Elongation

1) RNA polymerase covalently joins the nucleotides together. The two extra phosphates are released (provides energy)
2) Transcription occurs in a 5’ → 3’ direction (antisense strand)

8

Termination

At the terminator site, RNA polymerase is detached and the RNA sequence is released (and the DNA rewinds).

9

Modifications of mRNA after transcription in eukaryotic cells (3)

1) Capping
2) Polyadenylation
3) Splicing

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Capping

1) Addition of a methyl group to the 5’-end of the transcribed RNA
2) The methylated cap provides protection against degradation by exonucleases
3) Allows the transcript to be recognized by the cell’s translational machinery

11

Polyadenylation

1) Addition of a long chain of adenine nucleotides (a poly-A tail) to the 3’-end of the transcript
2) The poly-A tail improves the stability of the RNA transcript and facilitates its export from the nucleus

12

Splicing

Non-coding regions within genes are removed

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Introns

Non-coding regions in genes

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Exons

Coding regions of genes

15

Alternative Splicing

Exons can be selectively removed to form different proteins from the same gene

16

Regulators of transcription activity

1) Transcription factors
2) Regulatory proteins

17

Transcription factors

Form a complex with RNA polymerase at the promoter
(Without these factors RNA Pol cannot initiate transcription)

18

Regulatory proteins

Bind to DNA sequences outside of the promoter and interact with the transcription factors

19

Activator proteins

Bind to enhancer sites and increase the rate of transcription

20

Repressor proteins

Bind to silencer sequences and decrease the rate of transcription

21

The environment of a cell has an impact on gene expression

Chemical signals within the cell can trigger changes in levels of regulatory proteins or transcription factors in response to stimuli

22

Examples of organisms changing their gene expression patterns in response to environmental changes

1) Hydrangeas change colour depending on the pH of the soil
2) The Himalayan rabbit produces a different fur pigment depending on the temperature
3) Humans produce different amounts of melanin depending on light exposure

23

Nucleosomes and Transcription Regulation in Eukaryotes

Histones proteins have protruding tails that determine how tightly the DNA is packaged within nucleosomes

24

Acetylation and DNA package

Makes DNA less tightly packed

25

Methylation and DNA package

Makes DNA more tightly packed

26

Types of Chromatin

1) Heterochromatin (Supercoiled | No transcription)
2) Euchromatin (Loosely packed | Transcription)

27

DNA Methylation

1) Increased methylation of DNA decreases gene expression
2) Genes that are not transcribed tend to exhibit more DNA methylation than genes that are actively transcribed
3) Environmental factors (e.g. diet, pathogen exposure, etc.) influences DNA methylation.