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
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 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
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
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
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
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
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
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