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Translation
Translation converts the order of the nucleotides of a gene into the order of amino acids in a protein
The rules that govern translation are called the genetic code
mRNAs are the “blueprint” copies of nuclear genes mRNAs are “read” by a ribosome in three-nucleotide
units, termed codons Each three-nucleotide sequence codes for an amino
acid or stop signal
Ribosomes
The protein-making factories of cells
Sites play key roles in
translation
They use mRNA to direct the assembly of a protein
A ribosome is made up of two subunits
Each of which is composed of proteins and rRNA
Transfer RNA
tRNAs bring amino acids to the ribosome
They have two business ends Anticodon which is
complementary to the codon on mRNA
3’–OH end to which the amino acid attaches
Hydrogen bonding causes
hairpin loops
3-D shape
Making the Protein
mRNA binds to the small ribosomal subunit
The large subunit joins the complex, forming the complete ribosome
mRNA threads through the ribosome producing the polypeptide
How translation works
The process continues until a stop codon enters the A site
The ribosome complex falls apart and the protein is released
Play Protein Synthesis
Architecture of the Gene
In eukaryotes, genes are fragmented They are composed of
Exons – Sequences that code for amino acids Introns – Sequences that don’t
Eukaryotic cells transcribe the entire gene, producing a primary RNA transcript This transcript is then heavily processed to produce the
mature mRNA transcript This leaves the nucleus for the cytoplasm
Different combinations of exons can generate different polypeptides via alternative splicing
Processing eukaryotic mRNA
Protect from degradation and facilitate translation
Play How Spliceosomes Process RNA
Cytoplasm
Nuclear membrane
DNA
RNA polymera
se
PrimaryRNA transcript
5’3
’5’
3’
1. In the cell nucleus, RNA polymerase transcribes RNA from DNA
5’3
’CapPoly-A
tail
2. Introns are excised from the RNA transcript, and the remaining exons are spliced together, producing mRNA
Introns3’
mRNA
Exons
5’
mRNA
Cap
Nuclear pore
Poly-A tail
3. mRNA is transported out of the nucleus. In the cytoplasm, ribosomal subunits bind to the mRNA
Small ribosomal
subunit
Large ribosomal
subunit
tRNA
Amino
acid
4. tRNA molecules become attached to specific amino acids with the help of activating enzymes. Amino acids are brought to the ribosome in the order dictated by the mRNA.
5. tRNAs bring their amino acids in at the A site of the ribosome. Peptide bonds form between amino acids at the P site, and tRNAs exit the ribosome from the E site.
5’
3’
Ribosome
Ribosome moves toward 3’ end
6. The polypeptide chain grows until the protetin is completed.
Completed polypeptide
7. Phosphorylation or other chemical modifications can alter the activity of a protein after it is translated.
Play Control of Gene Expression
How protein synthesis works in eukaryotes
Architecture of the Gene
Most eukaryotic genes exist in multiple copies Clusters of almost identical sequences called multigene families
As few as three and as many as several hundred genes
Transposable sequences or transposons are DNA sequences that can move about in the genome They are repeated thousands of times, scattered randomly about the
chromosomes
Turning Genes Off and On
Genes are typically controlled at the level of transcription
In prokaryotes, proteins either block or allow the RNA polymerase access to the promoter Repressors block the promoter Activators make the promoter more accessible
Most genes are turned off except when needed
The lac Operon
An operon is a segment of DNA that contains a cluster of genes that are transcribed as a unit
The lac operon contains Three structural genes
Encode enzymes involved in lactose metabolism Two adjacent DNA elements
Promoter Site where RNA polymerase binds
Operator Site where the lac repressor binds
The lac Operon
In the absence of lactose, the lac repressor binds to the operator RNA polymerase cannot access the promoter
Therefore, the lac operon is shut down
The lac Operon
In the presence of lactose, a metabolite of lactose called allolactose binds to the repressor
This induces a change in the shape of the repressor which makes it fall off the operator
RNA polymerase can now bind to the promoter
Transcription of the lac operon is ON
The lac Operon
What if the cell encounters lactose, and it already has glucose? The bacterial cell actually prefers glucose!
The lac operon is also regulated by an activator The activator is a protein called CAP
It binds to the CAP-binding site and gives the RNA polymerase more access to the promoter
However, a “low glucose” signal molecule has to bind to CAP before CAP can bind to the DNA
Play Combination of Switches
Enhancers
DNA sequences that make the promoters of genes more accessible to many regulatory proteins at the same time
Usually located far away from the gene they regulate
Common in eukaryotes; rare in prokaryotes
Mutation
The genetic material can be altered in two ways Recombination
Change in the positioning of the genetic material
Mutation Change in the content
of the genetic materialBithorax mutant
Mutation
Mutation and recombination provide the raw material for evolution
Evolution can be viewed as the selection of particular combinations of alleles from a pool of alternatives The rate of evolution is ultimately limited by the rate at which these
alternatives are generated
Mutations in germ-line tissues can be inherited
Mutations in somatic tissues are not inherited They can be passed from one cell to all its descendants
Kinds of Mutation
Mutations are caused in one of two ways
Errors in DNA replication Mispairing of bases by DNA polymerase
Mutagens Agents that damage DNA
Kinds of Mutation
The sequence of DNA can be altered in one of two main ways
Point mutations Alteration of one or a few bases
Base substitutions, insertion or deletion
Frame-shift mutations Insertions or deletions that throw off the reading frame
Kinds of Mutation
The position of genes can be altered in one of two main ways
Transposition Movement of genes from one part of the genome to
another Occurs in both eukaryotes and prokaryotes
Chromosomal rearrangements Changes in position and/or number of large segments
of chromosomes in eukaryotes
Mutation, Smoking and Lung Cancer
Agents that cause cancer are called carcinogens These are typically mutagens
The hypothesis that chemicals cause cancer was first advanced in the 18th century Many investigations since then have determined that chemicals can
cause cancer in both animals and humans For example, tars and other chemicals in cigarette smoke can
cause cancer of the lung