Transcript LECTURE #24: RNA and Transcription

RNA and
Transcription
Lecture #24
Honors Biology
Ms. Day
Gene expression
 DNA contains…
 information
in a specific sequence (order)
of nucleotides along 2 DNA strands

Leads to specific traits
 Gene expression includes two stages
 Transcription: DNA  RNA
 “transcribe” = to copy into another
form
 Translation: RNA  polypeptide
 “translate” = to change into another
language
THINK:
Translation
happens
“LATER”
What are the
characteristics of RNA?
 Ribose Nucleic Acid
 Single stranded
 Made of sugars (called ribose), phosphate groups
and nitrogen bases
 Backbone= alternating ribose sugar/phosphates
held together by PHOSPHODIESTER BONDS
 Made of RNA nucleotides
 Contains bases:
 (A) Adenine
 (G)Guanine
 (C) Cytosine
 (U) Uracil (replaces Thymine)
Type of RNA
Function Job
mRNA
Brings message
from DNA (DNA
instructions) to
ribosome to
make protein
(messenger RNA)
tRNA
(transfer RNA)
Transfers/moves
amino acids to
ribosomes
Makes up
rRNA
ribosomes along
(ribosomal RNA)
with proteins
Picture
The Ribosome
 Part of cell where translation (protein
synthesis) occurs
 Where proteins are actually made
Basic Principles of
Transcription and Translation
 Transcription (uses mRNA)
 the
synthesis of RNA from DNA
 Produces messenger RNA (mRNA)
 Occurs in the nucleus of eukaryotes and
nucleid region of prokaryotes
 Translation (uses mRNA, tRNA, rRNA)
 actual synthesis of a polypeptide
 mRNA  polypeptide  protein
 Occurs on ribosomes
In prokaryotes, transcription and
translation occur together
TRANSCRIPTION
DNA
mRNA
Ribosome
TRANSLATION
Polypeptide
Prokaryotic cell. In a cell lacking a nucleus, mRNA
produced by transcription is immediately translated
without additional processing.
In eukaryotes, pre mRNA
transcripts are modified (changed) before
becoming true “mature” mRNA
Nuclear
envelope
“Transcript”
is a fancy
word for
“message”
DNA
TRANSCRIPTION
Pre-mRNA
RNA PROCESSING
mRNA
Ribosome
TRANSLATION
(b)
Polypeptide
Eukaryotic cell. The nucleus provides a
Separate compartment for transcription.
The original RNA transcript, called
pre-mRNA, is processed in various
ways before leaving the nucleus as mRNA.
How is Protein Made?
 Cells are controlled by a cellular
chain of command
 DNA RNA protein

Called the “Central dogma of biology”
 What are proteins made out of?
 Amino

acids
There are 20 different amino acids
 building blocks of proteins

All living things use the same 20
amino acids to make proteins!!!
The Genetic Code
 It’s a table used that TRANSLATES
RNA nucleotides (or mRNA
“words”) into one of the 20 amino
acids
 3 letter mRNA “word” = 1
amino acid
 There are 4 different RNA
“letters” that can be used to
make “words”  A, U, C, and G
Codons:
Triplets of Bases
 nonoverlapping base triplet is a codon
3 letter mRNA “words” = codon
 FOUND ONLY ON mRNA
 Codons must be read in the correct
order
 For specified polypeptide to be
produced
 Always read in the 5’  3’ direction

THE
GENETIC
CODE
64
CODONS
TOTAL
During transcription, a gene
determines the sequence of bases
along length of mRNA.
Gene 2
DNA
molecule
Gene 1
Gene 3
DNA strand
(template)
3
A
C
C
A
A
A
C
C
G
A
G
T
U
G
G
U
U
U
G
G
C
U
C
A
5
TRANSCRIPTION
mRNA
5
3
Codon
TRANSLATION
Protein
Trp
Amino acid
Phe
Gly
Ser
Transcription
DNA  RNA
mRNA is made by RNA
polymerase
breaks DNA strands apart
(breaks H bonds btw
bases) and hooks
together RNA nucleotides
Transcription
Follows same DNA base-
pairing rules, except in
RNA, uracil substitutes
for thymine
A = U (T on DNA = A in RNA)
C = G
Synthesis of an RNA
Transcript
 Initiation


DNA strands unwind
RNA polymerase binds DNA at a special place called a
promoter

“TATA box” = start signal on DNA promoter
 Determines which strand is used as
template  only 1 side is used at a time!
 Elongation
RNA polymerase makes mRNA by reading DNA’s
message
 Termination
 mRNA transcript is released at terminator signal
 RNA polymerase detaches from the DNA

Transcription Animation
 http://www.stolaf.edu/people/giannini/flashani
mat/molgenetics/transcription.swf
 http://wwwclass.unl.edu/biochem/gp2/m_biology/animati
on/gene/gene_a2.html
Another good animation
 http://www.biostudio.com/d_%20Transcription
.htm
Pre-mRNA Modification:
pre-mRNA  mature mRNA
 Eukaryotic cells modify (process)
mRNA after transcription
 First mRNA made is called premRNA
 Pre-mRNA  made into mRNA
2
things have to happen…
NOT IN PROKARYOTES
1. Alteration of mRNA
Ends
 Each end of a pre-mRNA is capped
 5
end gets a GTP cap
 3 end gets a poly-A tail
LOTS of adenine
nucleotides
added to the 3 end
GTP
added to the 5 end
SCRIPTION
OCESSING
DNA
Pre-mRNA
5
mRNA
Ribosome
TION
Polypeptide17.9
Figure
Protein-coding
segment
G P P P
5 Cap
AAA…AAA
Start codon
Stop codon
Poly-A tail
 The function of the 5’ cap is:
 prevents
mRNA degradation
 helps attach to the ribosome
 Function of the 3’ tail (Poly A cap):
 prevents mRNA degradation
 Exports mRNA from nucleus into
cytoplasm
2. RNA Splicing
 RNA splicing
 Removes


Introns = non-coding regions
Exons = coding regions that EXIT nucleus
Pre-mRNA
TRANSCRIPTION
RNA PROCESSING
introns and joins exons
DNA
5
Intron
Intron
3
5 Cap
Poly-A tail
Exon
Exon
Exon
Pre-mRNA
Coding
segment
mRNA
Ribosome
Mature mRNA
Introns cut out and
exons spliced together
TRANSLATION
Polypeptide
Figure 17.10
5 Cap
Poly-A tail
RNA Animations #1
www.geneticsplace.com
 CHAPTER 14: RNA Animations


RNA Biosynthesism
RNA Production in Eukaryotes
RNA Splicing
RNA Splicing Animations
#2
 http://highered.mcgraw-
hill.com/sites/0072437316/student_view0/cha
pter15/animations.html
 http://highered.mcgrawhill.com/sites/0072437316/student_view0/cha
pter15/animations.html#
 http://media.pearsoncmg.com/bc/bc_campbel
l_biology_7/media/interactivemedia/activities/l
oad.html?19&B