Transcript ap ch 17 powerpoint - Pregitzersninjascienceclasses

Toe-Tapping Transcription
and Translation
From Gene to Protein...
Chapter 17
I. Background Information
 DNA
contains information for how to
“build” an organism
 Variations in genes make different
phenotypes
 Proteins are the links between genotype
and phenotype
 Proteins are one or more polypeptide
 One gene codes for one polypeptide
II. The Big Picture
Flow of Information
Transcription
Genes (DNA)
mRNA
Nucleus
A.
II. The Big Picture
Translation
mRNA
Protein/(polypeptide)
Ribosome
II. The Big Picture

B. Difference Between DNA and RNA
SUGAR
BASES
SIZE
LOCATION
DNA
Deoxyribose
A,T,C,G
Double Strand
Nucleus
RNA
Ribose
A,U,C,G
Single Strand
Leaves nucleus
To cytoplasm
II. The Big Picture
C.
Prokaryotic vs. Eukaryotic

Prokaryotic have no nucleus, so
transcription and translation happen
simultaneously
Eukaryotic have processes separated,
one in nucleus and one in cytoplasm

Genetic Code
DNA contains only 4 bases
 There are 20 amino acids
 To make all 20, you must have
combinations of at least 3 nucleotide
bases
 Triplet Code: DNA contains 3 letter codes
= codon that make each amino acid

III. Transcription
A.
The Overview
1.
DNA opens up
mRNA reads template side (only one
side of DNA is used)
Don’t forget that Uracil replaces T
2.
3.
III. Transcription
B.
The Details
1.
RNA polymerase opens the two strands of DNA and
hooks RNA nucleotides as they base pair along the
DNA template
Works only in the 5’ to 3’ direction
Starts at a promoter sequence.
Ends at a terminator sequence
Transcription Unit = gene + promoter + terminator
As it elongates, strand detaches from DNA template.
DNA closes behind it.
2.
3.
4.
III. Transcription

Initiation
Promoter – attachment point for RNA
polymerase, determines which side of DNA is
template
 In Eukaryotes – promoter includes a TATA
box and transcription factors that help RNA
polymerase to bind

III. Transcription

Elongation
RNA polymerase moves along DNA and
unwinds it
 Brings in correct RNA bases to match DNA
template
 RNA peels away from DNA template
 More than one RNA strand can be made at
one time

III. Transcription

Termination
Bacteria – when RNA polymerase reaches
the termination signal it detaches from DNA
and releases the transcript as mRNA
 Eukaryotes – RNA polymerase reaches
polyadenylation sequence AAUAAA and
proteins cut off pre-mRNA from the DNA
template

III. Transcription
C.
Alteration of mRNA ends
(eukaryotes)
1.
5’ end gets a guanine cap (5’ cap)
for protection and attach signal for
ribosome.
3’ end gets a poly(A) tail (50-250
adenines). This protects, is used
for attachment, and aides in
transport.
2.
III. Transcription
D.
RNA splicing
1.
Introns = non-coding sections
Exons = coding sections
Introns get spliced before the mRNA
goes to the cytoplasm
Spliceosome, made up of small nuclear
ribonucleoproteins (snRNPs) and
proteins, cut out the introns.
2.
3.
III. Transcription
E.
Why Introns?

Different splicing patterns make different
polypeptides
Introns may regulate gene activity
Alternative splicing may make different
sections of a protein


IV. Tricky Translation
A.
The Basics

mRNA (all spliced and modified) goes out of
nucleus to cytoplasm to make a protein in the
ribosome
mRNA is read in groups of three bases called
CODONS.
tRNA transfers amino acids from cytoplasm
sea to the ribosome
the ribosome connects the amino acids
together



B. Details on tRNA
Each tRNA carries a specific amino acid
at one end
 At the other end is a nucleotide triplet
called an anticodon. This base pairs with
the mRNA.
 Made in nucleus, goes to cytoplasm
 Can be used repeatedly
 Short single strand of nucleotides

B. Details on tRNA
Folds into 3D shape (we love H-bonds)
 Wobble - some tRNA’s have anticodons
that can recognize two or more codons,
these involve switches in the third-position
only.
 Aminoacyl-tRNA synthetase - Enzyme
that joins amino acid to tRNA. 20
varieties, 1 for each amino acid.

C. Rockin’ Ribosomes
1.
2.
3.
Made up of a large and small subunit
which are made of proteins and
ribosomal RNA
Subunits are made in nucleolus
Place where mRNA codons and tRNA
amino acids come together to make a
protein
C. Rockin’ Ribosomes
4.
5.
6.
P-site: holds the tRNA carrying the
growing polypeptide chain
A-site: holds the tRNA carrying the
next amino acid to be added to the
chain
E-site: Discharged tRNA’s leave the
ribosome from here.
D. Building a Polypeptide
1.
Initiation

small ribosomal subunit binds to molecule of
mRNA at 5’ end
An initiator tRNA (methionine-AUG) binds to Psite
large subunit attaches to make functional
ribosome. A-site is ready to get next amino
acid
GTP, form of energy, is expended to put the
ribosome together



D. Building a Polypeptide
2.
Elongation

Codon recognition occurs as mRNA in the Asite of ribosome bonds with anticodon of tRNA
(with amino acid). This requires GTP.
Amino acid in P-site binds to amino acid in Asite with a peptide bond to build the protein.
Translocation - ribosome moves tRNA in A-site
to P-site. tRNA in P-site is released. Building
continues.


D. Building a Polypeptide
3.
Termination

Keeps going until stop codon in mRNA
reaches A-site of ribosome
UAA, UAG, UGA are stop codons
Release factor binds to stop codon in Asite. Water added instead of an amino
acid


E. Other Fun Stuff
1.
Polyribosomes

2.
Many ribosomes can work one strand of
mRNA to make bulk protein
Becoming Functional by Folding


Folds into secondary, tertiary, and
quaternary structures
Post translational modifications include
attachment of sugars, lipids, phosphates,
etc.
V. Mucky Mutations
A.
Overview
1.
Changes in genetic make-up of a cell
Point mutations-Chemical change in just
one base pair
In gametes, it can go to offspring (sicklecell anemia)
2.
3.
B. Substitution
1.
2.
3.
Replacement of one nucleotide and its
partner in the complementary DNA
strand with another pair of nucleotides
Missense Mutation-altered codon still
codes for an amino acid, so it makes
sense, just wrong sense
Nonsense Mutation-changes amino acid
codon to a stop signal – almost always
makes nonfunctional protein
C. Insertions or Deletions
1.
2.
3.
Additions or losses of nucleotide pairs in
a gene
Throws off the triplet reading (frame shift)
Produces nonfunctional proteins
D. Why/How Mutations?
1.
2.
3.
Errors during DNA replication or repair
(Spontaneous mutations)
Mutagens, like physical and chemical
stuff, can interact with DNA and mutate it
Mutagens include UV light, X-rays, and
chemicals
What is a Gene?

A gene is a region of DNA that can be
expressed to produce a final functional
product that is either a polypeptide or an
RNA molecule