Download CHAPTER 10 - Protein Synthesis The DNA genotype is expressed

CHAPTER 10 - Protein Synthesis
The DNA genotype is expressed as proteins, which provide the molecular basis for
phenotypic traits
• The information constituting an organism’s genotype is carried in the sequence
of bases in DNA
Figure 10.9
The DNA is transcribed into RNA, which is translated into the polypeptide
•
Studies of inherited metabolic disorders first suggested that phenotype is
expressed through proteins
Figure 10.13a
Transcription produces genetic messages in the form of RNA
RNA Transcription
• Process in which
•
During transcription only 1 DNA stand serves as the template or pattern from
which RNA is formed.
Figure 10.13b
• In transcription, the DNA helix unzips
• RNA nucleotides line up along one strand of the DNA following the basepairing rules
• The single-stranded messenger RNA peels away and the DNA strands rejoin
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•
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Initiation
The enzyme RNA polymerase attaches to the promoter site on the DNA
Promoter –
– tells RNA polymerase to start transcription and which of the two DNA
strands to transcribe
•
•
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Elongation
RNA nucleotides attach to the free DNA nucleotides by hydrogen bonds one
at a time
As RNA synthesis continues the growing RNA strand peels away from the
DNA and the DNA strands rejoin
•
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Termination
RNA polymerase reaches the terminator.
Terminator –
•
The RNA polymerase detaches from the DNA and the RNA molecule is
complete
Eukaryotic RNA is processed before leaving the nucleus
• Noncoding segments called introns are spliced out
• The coding segments called exons are joined together
• A cap and a tail are added to the ends
Explain RNA Transcription in your own words
• RNA polymerase
RNA processing
• Initiation
intron
• Elongation
exon
• Termination
cap
• Promoter
tail
• terminator
Genetic information written in codons is translated into amino acid sequences
• The “words” of the DNA “language” are triplets of bases called codons
– The codons in a gene
Figure 10.11
The genetic code is the Rosetta stone of life
• Virtually all organisms share the same genetic code
•
An exercise in translating the genetic code
Translation
• The process in which a polypeptide is synthesized using the genetic
information encoded on an mRNA molecule
• The following are needed for translation to occur
– mRNA
-
Codon – a sequence of 3 bases on mRNA that specifies a specific amino
acid that will be added to the polypeptide chain
Figure 10.15
Transfer RNA molecules serve as interpreters during translation
• In the cytoplasm, a ribosome attaches to the mRNA and translates its message
into a polypeptide
•
•
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Each tRNA molecule has a triplet anticodon on one end and an amino acid
attachment site on the other
tRNA (transfer RNA)
A tRNA molecule is composed of
– A single strand of RNA (about 80 nucleotides)
– A loop at one end that contains the anticodon
– Anticodon –
– At the opposite end of the loop is a site where an amino acit can attach
3. Amino acids
• Located in the cytoplasm
• Synthesized from other chemicals or obtained from food
Figure 10.16
Ribosomes build polypeptides
•
•
Ribosomes
Consists of 2 subunits each made up of proteins and ribosomal RNA (rRNA)
– Small subunit – has binding site for mRNA
– Large subunit – has binding site for tRNA
Figure 10.17
An initiation codon marks the start of an mRNA message
Figure 10.18
• mRNA, a specific tRNA, and the ribosome subunits assemble during initiation
Elongation adds amino acids to the polypeptide chain until a stop codon terminates
translation
• The mRNA moves a codon at a time relative to the ribosome
– A tRNA pairs with each codon,
Figure 10.19
Steps of Translation
• Initiation
• mRNA binds to the ribosome
•
The first amino acid (methionine) is brought to the ribosome by the tRNA
•
•
Elongation
Amino acids are added one by one to a growing polypeptide chain
•
Termination
•
The completed polypeptide is released
Modification of the polypeptide
Endoplasmic reticulum
• Collects proteins made by the ribosomes
• Packages them into vesicles
Golgi apparatus
• Proteins are altered, packaged into vesicles, and transported to different parts
of the cell or exported out of the cell
Figure 10.20
• Summary of transcription and translation
Review: The flow of genetic information in the cell is DNA→RNA→protein
• The sequence of codons in DNA spells out the primary structure of a
polypeptide
– Polypeptides form proteins that cells and organisms use
Describe the process of translation. Include the following:
• mRNA
termination
• tRNA
start codon
• Ribosome
P site
• Amino acid
A site
• Initiation
stop codon
• Elongation
Modification
Mutations can change the meaning of genes
• Mutations are changes in the DNA base sequence
– These are caused by errors in
– The change of a single DNA nucleotide causes sickle-cell disease
Figure 10.21
•
Types of mutations
Figure 10.22
Types of Mutations
There are 2 general categories of mutations:
• Base substitution
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Can result in no change in the protein
An insignificant change
– The altered amino acid has no effect on the function of the protein
A change that is crucial to life of the organism
– The altered amino acid has an effect on the function of the protein
Base insertions or deletions
Often have disastrous effects
– The nucleotide sequence following the change alters the genetic message
(reading frame)
Mutations are Useful
Mutations are useful because they
• Provide diversity that allows evolution by natural selection to occur
• Essential tool for geneticists
• Create different alleles needed for genetic research