Law: Gene Expression

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Amino Acids

1. sub-units of proteins
2. 20 different types
3. 20ⁿ possibilities of proteins

Process of Gene Expression

1. DNA sequence→ Transcription → Translation

Where are proteins made?

in cytoplasm

Transcription

1. transcribe gene sequence into mRNA sequence (pre-mRNA)
2. occurs in the nucleus
3. pre-mRNA is written in 5'-3' direction, copied from 3'-5' DNA strand

Antisense Strand

DNA strand running in 3'-5' direction used as template for pre-mRNA synthesis.

Sense Strand

1. DNA strand running in 5'-3' direction that is not used for pre-mRNA synthesis.
2. pre-mRNA strand looks really similar to this strand.

Initiation of Transcription

1. Transcription factors bind to promoter region and TATA box
2. RNA polymerase II bind with transcription factors
3. RNA polymerase moves along DNA toward gene
4. Transcription bubble forms
5. RNA is made denovo off of DNA template

Promoter Region

DNA sequence that marks the beginning of gene expression

TATA box

DNA sequence that acts on gene by turning it on and off for gene expression.

Differences between transcription and DNA replication

1. initiation does not require an RNA primer, RNA polymerase can open DNA and bind nucleotides
2. only genes are transcribed rather than whole DNA sequence.
3. Thymine is replaced for Uracil.
4. a single RNA strand is made rather than a double helix.

Elongation of Transcription

1. RNA polymerase II moves along DNA template
2. transcription bubble moves
3. complementary base pairing continues
4. ribonucleotides join (phosphodiester linkage)
5. pre-mRNA is made 5'-3' direction

Termination of Transcription

1. RNA polymerase II hits terminator (stop) sequence
2. RNA polymerase II falls off DNA template strand
3. pre-mRNA is released
4. DNA strands will reform h bonds with each other.

mRNA processing

1. transforms pre-mRNA so that it can move from nucleus to cytoplasm
2. 5' cap is added (m7-Gppp - energy molecule) - allows mRNA to bind to ribosomes
3. cap is a modified guanine molecule
4. introns are cut out and exons are spliced together
5. 3' poly A tail is added - 100-1000s of A's are added to slow mRNA degradation and to export mRNA strand

mRNA degradation

1. mechanism to prevent continuous protein synthesis (only made when needed)
2. pan RNAse will degrade tail of mRNA at 3' end in the cytoplasm

Introns

DNA sequence present in some genes that are transcribed but are removed during mRNA processing and therefore are not present in mature mRNA.

Exons

DNA codons that are transcribed, joined to other exons during mRNA processing and translated into amino acids.

Alternative Splicing

1. exons can be retained or removed during splicing, allowing processed mRNA to contain different combinations of exons
2. occurs in 95% of human genes

Characteristics of Genetic Code

1. universal
2. triplet codon
3. non-overlapping
4. degenerate - redundant
5. genetic code table

STOP codons

1. UAG
2. UAA
3. UGA
tells ribosome to stop adding AA

START codon

1. AUG, also methionine
2. all mRNA strands start to AUG to begin adding AA
3. methionine is always the first AA in polynucleotide

tRNA

1. small RNA molecule that contains a binding site for a specific type of amino acid and has a three-base sequence (anticodon) that recognizes a specific base sequence.
2. amino acid is on 3' end of tRNA

Anticodon

complementary codon of mRNA codon that will H bond.

Polyribosomes

multiple ribosomes are used to make one protein because mRNA degrades fairly quickly.

Ribosomes

1. only come together during translation
2. contains small and large subunits
3. made of rRNA
4. lives in cytoplasm

Initiation of Translation

1. mRNA binds to small ribosome subunit
2. start codon of mRNA is recognized (AUG)
3. tRNA anticodon (UAC) binds to start codon
4. methionine is first amino acid of chain
5. large ribosome subunit attaches

GTP

1. energy source for polypeptide production
2. breaks to form GDP giving energy for amino acid linkage

Elongation of Translation

1. intact ribosome reads mRNA in 5'-3' direction
2. moves one codon at a time
3. new tRNA binds to codon
4. adjacent AA are joined by peptide bond
5. first tRNA releases its AA and leaves
6. process continues
7. Large ribosome acts like an enzyme by peptide bonding amino acids together.

Termination of Translation

1. stop codon is reached - UAA, UAG, UGA
2. ribosome subunits come apart
3. mRNA is released
4. polypeptide is released

Structures of Polypeptides

1. 1° - polypeptide chain
2. 2° - pleated sheets or coils due to H bonds between AA
3. 3° - helical or pleated sheet (most proteins are in this structure)
4. 4° - physical association of 2 or more polypeptides (functional proteins) ie) hemoglobin

Chaperone Proteins

1. proteins that help newly synthesized polypeptides to fold correctly
2. attach to polypeptide and makes sure it folds correctly.
3. prions can cause protein mutations - ie) mad cow disease

Post Translational Modifications

1. lead to many different functional proteins
2. adds carbohydrates, phosphates, and/or lipids
3. proteolysis (cutting up 1 long protein strand into smaller proteins) and protein interactions (formation of 4° structure)

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