Transcript Chapter 17 Power Point

Chapter 17
AP Biology
From Gene to Protein
Central Dogma of Molecular
Biology
DNA  RNA Protein
What is RNA?
• Contains the bases A, C, G, and U instead of T
• single-stranded (often folds onto itself)
• Three types of RNA: messenger RNA (mRNA),
transfer RNA (tRNA) and ribosomal RNA
(rRNA)
Protein Synthesis
1. Transcription - DNA message is transcribed
into mRNA and sent to ribosomes.
2. Translation - mRNA is translated into a Protein
by a ribosome.
DNA  RNA  Protein
Overview –
Eukaryotic Cell
Notice that
transcription and
translation occur
in different
places in a
eukaryotic cell.
Overview – Prokaryotic Cell
The Triplet Code
Note that the
bases of mRNA are
complementary to
the template
strand of DNA
The bases of mRNA
are read in groups
of three – each
group is called a
codon
From mRNA to Amino Acids
• The mRNA base triplets are called codons
• mRNA is written in the 5’ to 3’ direction
• The codons code for each of the 20 amino
acids
• The genetic code is redundant
– Different codons code for the same amino acid
The Genetic
Code
The three bases of
an mRNA codon
code for the 20
amino acids that
are the subunits of
proteins
The amino acids
are designated by
three letters
Universal Nature of the Genetic Code
RNA Processing
• In eukaryotes, transcription results in premRNA that needs processing before it leaves
the nucleus.
Processing of Pre-mRNA
• Addition of a 5’ cap
– A modified form of a guanine nucleotide
• Addition of a poly-A tail
– 50-250 adenine (A) nucleotides are added
– This is referred to as the poly-A tail
• RNA splicing
– Editing of the initial strand of mRNA (a cut and
paste job)
Addition of 5’ cap and poly-A tail
5’ cap and poly-A tail
• Both facilitate export of mRNA from the
nucleus
• Both help protect mRNA from degradation by
enzymes
• Both facilitate the attachment of mRNA to the
ribosome
RNA Splicing
• Only takes place in eukaryotic cells
• Large sections are spliced out; these are called
introns
• The sections that remain are called exons
– “exons EXIT the nucleus”
• These exons are spliced together to form the
final mRNA that will be translated
RNA Splicing
Alternative RNA Splicing
• Allows for different combinations of exons
• This results in more than one protein per gene
• This explains why we have fewer genes in our
genome than what was expected
• The human genome contains about 21,000
protein-encoding genes, but the total number
of proteins in human cells is estimated to be
between 250,000 to one million.
Transcription Animation
• https://www.youtube.com/watch?v=SMtWvD
bfHLo
17.4 Translation
• tRNA functions in transferring amino acids
from the cytoplasm to a ribosome
• rRNA complexes with proteins to form the two
subunits that make up ribosomes
tRNA
• Each type of tRNA is specific for a particular
amino acid
• At one end tRNA loosely binds the amino acid
and at the other end it has a nucleotide triplet
called the anticodon
• The anticodon allows it to pair specifically
with a complementary codon on mRNA
Structure of tRNA
Basic Concept of
Translation
Codons are the
triplet nucleotides
on mRNA
Anticodons are the
triplet nucleotides
on tRNA
Three Stages of Translation
• Initiation
– Begins with the start codon AUG (always!)
• Elongation
– Codon recognition
– Peptide bond formation (between 2 a.a.)
• Termination
– A stop codon is reached and translation stops
Initiation
Elongation
Termination
Translation Animation
• https://www.youtube.com/watch?v=TfYf_rPW
UdY
Folding of the Polypeptide
• Following release from the ribosome, the
polypeptide then folds to its specific
conformation (3d shape)
• Chaperonins are the proteins that help with
this folding process
• The first 20 amino acids of the polypeptide
serve as a signal peptide and act as a cellular
zip code, directing the polypeptide to its final
destination
Targeting Proteins to the ER
17.5 Point Mutations
• Mutations are alterations in the genetic material of the
cell caused by mutagens
• Point mutations are alterations of just 1 base pair in a
gene
– Base-pair substitution
• Silent mutations – have no effect on the encoded protein
• Missense mutations – change one amino acid to another;
might still code for the correct amino acid
• Nonsense mutations – change a regular amino acid codon
into a stop codon
– Insertions & deletions
• Frameshift mutation – codons are read in the wrong
“frame”
Base-Pair Substitution - Silent
Base-Pair Substitution - Missense
Base-Pair Substitution - Nonsense
Frameshift Mutation- Insertion
Frameshift Mutation - Deletion