Genes and Protein Synthesis
Chapter 7
One Gene-One Polypeptide Hypothesis• DNA contains all of
our hereditary information
• Genes are located in our DNA
• ~25,000 genes in our DNA (46 chromosomes)
• Each Gene codes for a specific polypeptide
Main Idea• Central Dogma– Francis Crick (1956)
Overall Process• Transcription – DNA to RNA
• Translation – Assembly of
amino acids into polypeptide
– Using RNA
DNA molecule
Gene 1
Gene 2
Gene 3
DNA strand
TRANSCRIPTION
RNA
Polypeptide
TRANSLATIONCodon
Amino acid
Key Terms • RNA transcription– Initiation,
Elongation, Termination
• TATA box • Introns, Exons• mRNA, tRNA, rRNA• Translation • Ribosome• Codon• Amino Acids• Polypeptide
DNA RNA
Double stranded Single stranded
Adenine pairs with Thymine Adenine pairs with Uracil
Guanine pairs with Cytosine Guanine pairs with Cytosine
Deoxyribose sugar Ribose sugar
DNA to Protein • Protein is
made of amino acid sequences
• 20 amino acids
• How does DNA code for amino acid?
DNA molecule
Gene 1
Gene 2
Gene 3
DNA strand
TRANSCRIPTION
RNA
Polypeptide
TRANSLATIONCodon
Amino acid
Genetic Code• Codon– Three letter code– 5’ to 3’ order– Start codon – Stop codon
• AA are represented by more than one codon
• 61 codons that specify AA
Amino acids • Abbreviated– Three letters
Transcription • DNA to RNA• Occurs in nucleus • Three process– Initiation – Elongation – Termination
RNA polymerase
DNA of gene
PromoterDNA Terminator
DNAInitiation
Elongation
TerminationGrowingRNA
RNApolymerase
Completed RNA
Initiation• RNA polymerase binds to DNA• Binds at promoter region
– TATA box• RNA polymerase unwinds DNA• Transcription unit
– Part of gene that is transcribed• Transcription factors bind to
specific regions of promoter • Provide a substrate for RNA
polymerase to bind beginning transcription
• Forms transcription initiation complex
Elongation • RNA molecule is
built– RNA polymerase
• Primer not needed• 5’ to 3’ direction • Template strand is
copied– 3’ to 5’ DNA
• Coding strand– DNA strand that is
not copied• Produces mRNA
– Messenger RNA • DNA double helix
reforms
Termination • RNA polymerase recognizes a termination sequence
– AAAAAAA (polyadenylation)• Nuclear proteins bind to string of UUUUUU on RNA• mRNA molecule releases from template strand
Post-Transcriptional Modifications• Pre-mRNA
undergoes modifications before it leaves the nucleus
• Poly(A) tail– Poly-A polymerase– Protects from RNA
digesting enzymes in cytosol
• 5’ cap– 7 G’s– Initial attachment
site for mRNA’s to ribosomes
• Removal of introns
Splicing the pre-mRNA• DNA comprised of – Exons • sequence of DNA or
RNA that codes for a gene
– Introns • non-coding
sequence of DNA or RNA
• Spliceosome– Enzyme that
removes introns from mRNA
Splicing Process• Spliceosome contains a handful of small
ribonucleoproteins– snRNP’s (snurps)
• snRNP’s bind to specific regions on introns
Alternative Splicing• Increases number and variety of proteins
encoded by a single gene• ~25,000 genes produce ~100,000 proteins
Translation• mRNA to protein • Ribosomes read
codons • tRNA assists
ribosome to assemble amino acids into polypeptide chain
• Takes place in cytoplasm
tRNA• Contains – triplet anticodon – amino acid
attachment site • Are there 61
tRNA’s to read 61 codons?
tRNA: Wobble Hypothesis • First two nucleotides of
codon for a specific AA is always precise
• Flexibility with third nucleotide
• Aminoacylation– process of adding an AA
to a tRNA – Forming aminoacyl-
tRNA molecule – Catalyzed by 20
different aminoacyl-tRNA synthetase enzymes
Ribosomes• Translate mRNA chains into amino acids• Made up of two different sized parts – Ribosomal subunits (rRNA)
• Ribosomes bring together mRNA with aminoacyl-tRNAs
• Three sites– A site - aminoacyl– P site – peptidyl– E site - exit
1 Codon recognition
Amino acid
Anticodon
AsiteP site
Polypeptide
2 Peptide bond formation
3 Translocation
Newpeptidebond
mRNAmovement
mRNA
Stopcodon
Translation process • Three stages– Initiation – Elongation – Termination
Initiation• Ribosomal subunits associate with mRNA • Met-tRNA (methionine)
– Forms complex with ribosomal subunits• Complex binds to 5’cap and scans for start codon (AUG) (scanning)• Large ribosomal subunit binds to complete ribosome • Met-tRNA is in P-site
Reading frame is established to correctly read codons
Elongation
• Amino acids are added to grow a polypeptide chain
• A, P, and E sites operate
• 4 Steps
Termination• A site arrives at a stop codon on mRNA – UAA, UAG, UGA
• Protein release factor binds to A site releasing polypeptide chain
• Ribosomal subunits, tRNA release and detach from mRNA
ba
Red object = ?
What molecules are present in this photo?
POLYSOME
Review • What is a gene?• Where is it
located?• What is the main
function of a gene?• Do we need our
genes “on” all the time?
• How do we turn genes “on” or “off”?
Regulating Gene Expression• Proteins are not
required by all cells at all times
• Regulated• Eukaryotes – 4 ways– Transcriptional (as mRNA
is being synthesized)– Post-transcriptional (as
mRNA is being processed)
– Translational (as proteins are made)
– Post-translational (after protein has been made)
Transcriptional regulation• Most common• DNA wrapped around histones keep gene promoters
inactive• Activator molecule is used (2 ways)– Signals a protein remodelling complex which loosen the
histones exposing promoter– Signals an enzyme that adds an acetyl group to histones
exposing promoter region
Transcriptional regulation• Methylation– Methyl groups are added to the cytosine bases in the
promoter of a gene (transcription initiation complex) – Inhibits transcription – silencing– Genes are placed “on hold” until they are needed– E.g. hemoglobin
Post transcriptional regulation• Pre-mRNA processing
– Alternative splicing• Rate of mRNA
degradation– Masking proteins used to
degrade mRNA – Translation does not occur
• Embryonic development
• Hormones – Casein – milk protein in
mammary gland– When casein is needed,
prolactin is produced extending lifespan of casein mRNA
Translational regulation• Occurs during
protein synthesis by a ribosome
• Changes in length of poly(A) tail– Enzymes add or
delete adenines – Increases or
decreases time required to translate mRNA into protein
– Environmental cues
Post-Translational Regulation• Processing
– Removes sections of protein to make it active
– Cell regulates this process (hormones)
• Chemical modification– Chemical groups are
added or deleted – Puts the protein “on hold”
• Degradation– Proteins tagged with
ubiquitin are degraded – Amino acids are recycled
for protein synthesis
Cancer• Lack regulatory mechanisms • Mutations in genetic code
(mutagens)– Probability increases over lifetime– Radiation, smoking, chemicals
• Mutations are passed on to daughter cells – Can lead to a mass of
undifferentiated cells (tumor)– Benign and malignant
• Oncogenes– Mutated genes that once served
to stimulate cell growth– Cause undifferentiated cell
division
Genetic Mutations• Positive and negative – Natural selection –
evolution – Cancer –death
• Small-Scale – single base pair– Point mutations • Substitution,
insertion/deletion, inversion
• Large-Scale – multiple base pairs
Small-Scale Mutations• Four groups – Missense, nonsense, silent, frameshift• Lactose, sickle cell anemia
– SNPs – single nucleotide polymorphisms• Caused by point mutations
Missense mutation• Change of a single base pair or group of base pairs• Results in the code for a different amino acid • Protein will have different sequence and structure
and may be non-functional or function differently
Nonsense mutation• Change in single base pair or group of base pairs • Results in premature stop codon • Protein will not be able to function
Silent Mutation• Change in one or more base pairs• Does not affect functioning of a gene• Mutated DNA sequence codes for same amino acid • Protein is not altered
Frameshift mutation• One or more nucleotides are inserted/deleted from a DNA
sequence• Reading frame of codons shifts resulting in multiple missense
and/or nonsense effects• Any deletion or insertion of base pairs in multiples of 3 does
not cause frameshift
Large-scale mutations • Multiple nucleotides,
entire genes, whole regions of chromosomes
Large-scale mutations • Amplification – gene
duplication– Entire genes are copied to
multiple regions of chromosomes
Large-scale mutations • Large-scale deletions – Entire coding regions of DNA are removed • Muscular Dystrophy
Large-scale mutations • Chromosomal translocation– Entire genes or groups of genes are moved from one
chromosome to another – Enhance, disrupt expression of gene
Large-scale mutations • Inversion– Portion of a DNA molecule reverses its direction in the
genome– No direct result but reversal could occur in the middle of
a coding sequence compromising the gene
Large-scale mutations • Trinucleotide repeat expansion– Increases number of repeats in genetic code – CAG CAG CAG CAG CAG CAG CAG CAG • Huntingtons disease
Causes of genetic mutations• Spontaneous mutations– Inaccurate DNA replication
• Induced mutations – Caused by environmental agent – mutagen – Directly alter DNA – entering cell nucleus – Chemicals, radiation
Chemical Mutagens • Modify individual
nucleotides– Nucleotides
resemble other base pairs
– Confuses replication machinery – inaccurate copying • Nitrous acid
• Mimicking DNA nucleotides – Ethidium bromide –
insert itself into DNA
Radiation - Low energy • UV B rays • Non-homologous end joining– Bonds form between adjacent nucleotides along DNA
strand – Form kinks in backbone – Skin cancer
Radiation – high energy • Ionizing radiation – x-ray, gamma rays • Strip molecules of electrons • Break bonds within DNA– Delete portions of chromosomes
• Development of tumors
Genomes and Gene organization• Human Body– 22 autosomal chromosomes– 1 pair of each sex chromosome (XX, YY)
Genomes and Gene organization• Components– VNTR’s–variable number tandem repeats (microsatellites)• Sequences of long repeating base pairs• TAGTAGTAGTAGTAG
– LINEs – long interspersed nuclear elements – SINEs – short interspersed nuclear elements – Transposons – small sequences of DNA that move about
the genome and insert themselves into different chromosomes
– Pseudogene – code is similar to gene but is unable to code for protein
Viruses• Not alive but can replicate themselves• Contain– DNA or RNA– Capsid – protein coat– Envelope – cell membrane
Virus • 4000 species of virus have been classified
HIV RNA Replication (Retrovirus) • Reverse
transcriptase to turn RNA into DNA
• Integrase incorporates into our genetic code
• Uses cells parts to make protein parts from mRNA
• genomic RNA
Influenza A• Viral RNA
replicated and transcribed for protein synthesis
Virus as Vectors• Transduction– Using a virus vector to insert DNA into a cell or
bacterium
Top Related