Genes, Chromosomes, &

Protein Synthesis

Genes, Chromosomes, Genes, Chromosomes, & &

Protein SynthesisProtein Synthesis

ByDr. Carmen Rexach

PhysiologyMount San Antonio College

DNA trivia• Each diploid human cell contains

approximately 2000mm of DNA

• The single largest human chromosome (remember…we have 23 pairs!) is 85mm in length and gets condensed into a structure 0.5μm in diameter and 10μm long when the cell divides

Eukaryotic Chromosomes

Highly condensed = about 100mg/ml

Chromatin and chromosomes• Chromatin

– Organized units of DNA and protein present in cell during all phases except cell division

– Euchromatin• Relatively decondensed chromatin accessible for

use– Heterochromatin

• Highly condensed chromatin that is transcriptionally inactive

• Chromosomes– Highly condensed chromatin present when the

cell is dividing

Chromatin vs Chromosomes

Chromatin structure• DNA + protein

– histone proteins = 5 major basic proteins with positive charge at neutral pH

– non-histone proteins = heterogenous, predominately acidic proteins with negative charge at neutral pH• Involved in DNA replication + gene

expression • Organized as nucleosomes

Nucleosomes

• Each nucleosome is composed of 146 bp of DNA wrapped around 8 histone proteins (octomer)

• Approximately 2 turns per nucleosome

• H1 histone anchors DNA to nucleosomecore

H1

octomer

Chromosome structure

Genome• Total genetic info in the DNA of a

typical cell in an organism• Human genome = 30,000–40,000

genes• Only 10-20% of the genes in the

human genome are expressed

Genes• A linear sequence of nucleotides that

codes for one product, usually protein

• There are millions of genes in DNA

Introns & Exons• Introns = intervening sequences

– non-coding regions in DNA– Used to be considered “junk”, now believed to

have effect on regulation and structure• Exons = expressed sequences

– coding regions in DNA

mRNA

DNA

protein

transcription

translation

Gene expression: Protein synthesis

RNA = protein synthesis• RNA nucleotides

– Phosphate– Sugar: Ribose– Nitrogen bases:

• purines = adenine and guanine (A, G)• pyrimidines = cytosine and uracil (C, U)

• three types: mRNA, tRNA, rRNA

mRNA = THE CODON

• Product of transcription• Two steps in eukaryotic cells

– pre mRNA– final mRNA

• triplet code = 3 nitrogen bases in linear sequence that code for one amino acid

A U

A

CUAAC

U

C A

AC C A A U C

AGGU GCPRE-mRNA

FINAL mRNA

Triplet code determines amino acids

AUG UUU GGU UCC GGU UGG CAU UUC

mRNA

DNA

tRNA = THE ANTI-CODON

• Made in nucleus• Circulate in cytoplasm associated with

an amino acid– triplet code on tRNA is complementary to the

mRNA codon

» where do the amino acids come from?

U A C

Met

A U GA G CU UA C

rRNA and Ribosomes

• Ribosomal components produced by nucleolus = rRNA + protein

• organelle involved in protein synthesis• Two subunits

tRNA binding sites

Catalytic enzyme

Large subunit

Small subunit

Protein synthesis

• Transcription– occurs in the nucleus– results in the production of mRNA

• Translation– occurs at the ribosomes in the cytosol– results in the production of protein

Steps of Transcription1. RNA polymerase breaks H

bonds in gene of interest only

2. RNA polymerase matches complementary RNA nucleotides with DNA base pairs on one side of DNA only producing pre-mRNA

3. pre-mRNA is released and hydrogen bonds in DNA are restored

Steps of Transcription4. pre-mRNA is edited

in the nucleus removing introns

• ends of exons spliced by snRNP’s to produce final mRNA

• Final mRNA leaves nucleus through the nuclear pore carrying the codon to the ribosome

snRNP’s = small, nuclear RNA particles, or “snirps”

Translation1. mRNA attaches to a ribosome2. tRNA matches its base pairs to codon on mRNA

following base pairing rules3. 2nd tRNA attaches, bringing the two amino acids at

the opposite ends into close proximity4. peptide bonds form between neighboring amino acids5. 1st tRNA falls off and returns to cytosol to pick up

more amino acids6. 2nd tRNA moves to position #1 as the ribosome

moves down, allowing a new tRNA to move into position #2.

7. process continues forming long polypeptide chain8. when translation is complete, mRNA dissociates from

the ribosome, and protein chain assumes its functional 3-D shape

Translation

Goes back to the cytosolto pick up more amino acids!

mRNA

Polyribosome

Post-transcriptional modification • Entire gene is copied as pre-mRNA• Pre-mRNA is spliced to produce final

mRNA• Final mRNA is used to produce protein

exon intron exon intron exon

exon exonexonintron

intron

Pre-mRNA

Final-mRNA

protein

Post-translational modification: inteins

• Some of these produce functional molecules with important roles!

exon exonexon intron intron Pre-mRNA

Protein made up of exeins and inteins

Exeins combined to form final product

Inteins are exised to form additional proteins

So…there are two ways

• Post-transcriptional modification– mRNA is modified in

the nucleus– Introns are removed– Exons are put back

together– mRNA leaves the

nucleus as final mRNA and is translated into protein at the ribosome

• Post-translational modification– mRNA leaves the

nucleus and goes to the ribosome

– mRNA is translated into protein

– After translation, inteins are removed

– Exeins are joined back together to make the final protein

mRNA

Post-translational processing

• Final protein produced by either method may be further processed– Cleavage of methionine from start site– Addition of carbohydrate or lipid

derivatives to amino acid side chains– Enzymatic cleavage of large protein into

smaller peptide chains

Regulation of protein synthesis

• Some genes continuously translated, others regulated

• Rate of protein synthesis regulated by:– 1) transcription of genes into mRNA– 2) Initiation of protein assembly on a ribosome– 3) degradation of mRNA

Mutations• Change in the DNA sequence• Role of mutagens• Types

– Point mutations– Frame shift mutations– Deletion mutations

• Effects– No change in cell function– Modify cell function – Lethal = cell death

Protein degradation• Restricts activity of protein by regulating

amount of given protein present in the cell• Rates of degradation differ for different

proteins• Degradation pathway

– Ubiquitin attaches to protein and directs it to a proteasome

– Proteasome unfolds protein and breaks it into small peptides

ub

ubub

ub ub

Protein secretion• Leader sequence (signal sequence) allows

for insertion of growing polypeptide chain into cisterna of ER, where it is modified, sorted, and sent to the Golgi

golgi

Plasmamembrane

Rough ER

Golgi

Cytoplasm

ECF