Chapter 12 Gene Expression Unlocking the secrets of DNA.

Chapter 12

Gene Expression

Unlocking the secrets of DNA

DNA is difficult to understand, even for the cell.

• The nucleotide sequence of DNA is the blueprint for building proteins, but the ribosomes responsible for making proteins cannot “read” DNA.

Ribonucleic Acid• 1) Messenger RNA

– Single strand, complimentary copy of DNA created in nucleus; contains triplet codons

• 2) Transfer RNA– Cloverleaf-shaped strand that

picks up amino acids and delivers them to the ribosome; contains triplet anti-codons

• 3) Ribosomal RNA– Makes up ribosomes; joins amino

acids together to create a growing protein chain

RNA vs DNA• RNA is single instead

of double strand• RNA has different

sugar (ribose)• RNA has uracil which

takes the place of thymine

• RNA moves out of nucleus & controls protein synthesis

RNADNA

RNApolymerase

Adenine (DNA and RNA)Cystosine (DNA and RNA)Guanine(DNA and RNA)Thymine (DNA only)Uracil (RNA only)

Transcription: the production of RNA from the DNA code

Protein Synthesis• 1. Transcription

– Occurs in nucleus– DNA makes a

complimentary copy in the form of M-RNA in a process similar to replication

– M-RNA moves out of nucleus and to ribosomes in the cytoplasm

Transcription

Benefits of transcription

• Transcribed copies of the DNA (in the form of RNA) are used instead of the original DNA.

• In eukaryotes, DNA in the cytoplasm is degraded but RNA is not.

How does transcription work?

1. DNA double helix must be separated at the hydrogen bonds between nitrogen bases.

2. Only one DNA strand is “read” by RNA polymerase.

3. RNA polymerase constructs an RNA polymer.

Building RNA polymer

• DNA (codes for) RNA

• A U

• T A

• C G

• G C

Click on image to play video.

What happens to the RNA once it is created?• In prokaryotes, the

RNA is immediately translated.

• In eukaryotes, the RNA is processed.– Introns removed– Exons joined together

RNA processing

• Introns (segments of useless genes) are removed, leaving on exons (expressed genes).

• 2. Translation– Occurs in cytoplasm– T-RNA delivers

amino acids to ribosomes & joins complimentary anti-codons, putting amino acids in proper order

– R-RNA forms peptide bonds to join amino acids into a protein molecule

– (T-RNA returns to pick up new amino acids)

Translation

Ribosomes use this decoding scheme to determine how to build the appropriate protein.

How does the decoding work?

• RNA: AUGCGAGGGAGAUUAUAGGAC

• Ribosomes read AUG – CGA – GGG – AGA – UUA – UAG – GAC.

• Each 3 nucleotide “word” is called a codon.

Try to decode AUG CGA GGG AGA UUA UAG GAC.

• Met – Arg – Gly – Arg – Leu - stop

What happens to Met–Arg–Gly–Arg–Leu–stop?

• The ribosomes create an amino acid polymer that is folded into a protein.

• The original DNA code, transcribed to RNA, instructs the cell to make a protein for a specific function.

What happens if there is a mutation in the original DNA?

• Point mutations: change of one nucleotide sequence

• This may or may not affect the amino acid sequence, depending upon where the mutation occurs in the DNA sequence.

Gene mutations• Point mutations

– Change one nucleotide or just a few nucleotides in a gene

– Examples: sickle cell anemia & cystic fibrosis

• Frame-shift mutations– The reading pattern is displaced and “shifts”

into new positions– Examples: duchenne Muscular Dystrophy

Frame shift mutations

• The cat and the rat ran far.

• The ca and the rat ran far.

• The caa ndt her atr anf ar.

t r

Mutations• Disadvantages:

– Abnormal, even lethal genes can be passed on to offspring

• Advantages:– New, stronger genes can be passed on to

offspring

Chromosomal mutations

• Deletion =

• Insertion =

• Inversion =

• Translocation =

A B C D E F G H

A B C D E G H

A B C D J E F G H

A B C D E G F H

G H W X Y ZA B C D E F

DNA 5’ A T G C C T G A A T G A 3’

3’ T A C G G A C T T A C T 5’ coding strand

mRNA A UG C C U G A A U G A codons

tRNA U A C G G A C U U A C U anti codons

amino

acids Met Pro Glu stop

protein

The Central Dogma theory of Biology

(DNA RNA protein cell functions)

Animation of translation

Click to automatically start animation.

Single Messenger RNA Strand

This is a ribosome.

Met


Met


Arg


MetArg


MetArg Gly


MetArg

Gly


MetArgGly


MetArg

Gly


MetArg

Gly Lys


MetArg

GlyLys

MetArgGly

Lys Stop

MetArgGly

LysStop

MetArgGlyLys

Stop

MetArgGly

LysStop

Protein Synthesis overview

Transcription: DNA codes for RNA

• RNA polymerase

• A U• T A• C G• G C

mRNA processing

• The RNA polymer made = messenger RNA (mRNA)

• DNA has “junk” genes. These are called introns (useless pieces of RNA made as a result of useless DNA).

• Introns are cut (“spliced”).

• Exons are remaining RNA nucleotides carrying the actual DNA code.

• Exons are translated.

Translation: RNA to amino acids

• Ribosomes “read” codons.

• AUG = start codon

• tRNA with anticodons have amino acids attached.

Translation

Ribosomes read until the codon that indicates STOP.

Amino acid chain

• The amino acid chain made in translation is then folded.

• Folded proteins = enzymes, pigments, etc.

• Proteins made = phenotype

Chapter 12 Gene Expression Unlocking the secrets of DNA.

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Transcript of Chapter 12 Gene Expression Unlocking the secrets of DNA.