Transcript DNA and Protein Synthesisx

DNA and Protein Synthesis
A blueprint for life
Protein Synthesis is divided into 2 parts in
Eukaryotes:Transcription and Translation
Transcription is the process
wherein the information coded
in DNA’s nucleotide sequence
is copied onto a strand of
Messenger RNA.
The language of the
information remains that of
nucleotide sequencing.
Transcription takes place
within the nucleus.
Translation is the process by
which the language of the
genetic information is changed
from the language of nucleotides
to the language of amino acids.
Translation takes place in the
cell’s cytoplasm using organelles
called ribosomes.
Translation involves all three
forms of RNA: Messenger RNA,
Transfer RNA, and Ribosomal
RNA.
Transcription happens in the nucleus.
The DNA strands separate and RNA
polymerase acts on only one of the
DNA strands.
RNA polymerase attaches to one
strand of DNA and adds
corresponding RNA nucleotides until
an m-RNA strand is completed.
The m-RNA strand that is built in the
nucleus is complementary to the
active DNA strand.
When the m-RNA strand is complete,
it leaves the nucleus through one of
the nuclear pores.
DNA makes all 3 types of RNA this
way.
DNA strand 1
RNA
polymerase
RNA nucleotides
DNA strand 2
Translation needs 3 forms of RNA.
In eukaryotes, the preliminary m-RNA
strand is processed in the nucleus,
where enzymes remove introns and
assemble exons to make a completed
m-RNA strand.
The m-RNA strand then leaves the
nucleus through a nuclear pore and
moves into the cytoplasm for the next
step in protein synthesis:Translation.
Translation is the actual making of a
polypeptide chain, which is directed
by m-RNA, but also needs r-RNA and
t-RNA.
m-RNA
Translation happens on ribosomes,
which are made of ribosomal RNA
(r-RNA). Ribosomes link amino
acids in an orderly way into
polypeptide chains.
Ribosomes are composed of two
subunits made of r-RNA. They snap
together over the m-RNA and have 2
binding sited for t-RNA.
Transfer RNA (t-RNA) picks up
amino acids and carries them to the
ribosome, where they are attached
together.
t-RNA
site
t-RNA
site
r-RNA
t-RNA
Amino acid
m-RNA binding site
The genetic code determines amino acid
sequences of polypeptide chains.
In the genetic code, which is the
same in all living things, nucleotide
triplets code for certain amino
acids.
The sequence of these triplets in an
m-RNA chain determines the order
of amino acids in a polypeptide
chain. It is called a codon.
There are 20 amino acids that are
found in the proteins of living
things. These triplets code for all of
them. There are many repeats that
have appeared in the code over the
billions of years that living things
have evolved.
1)
2)
3)
4)
5)
The dictionary of the genetic code
is usually shown as a series of mRNA codons.
Use the genetic code shown in
your textbook to determine which
amino acids are specified by the
following m-RNA codons.
AUG
CUU
GCA
UAU
GAC
Use the genetic code to identify
the three “stop” codons.
There are many types of t-RNA.
There are 64 kinds of t-RNA.
They all have the same basic
structure: a coiled chain of
nucleotides with a triplet of exposed
nitrogen bases at one end and an
attachment site for an amino acid at
the other end.
The triplet of exposed nitrogen bases
is called an anticodon.
When the anticodon binds with a
messenger RNA codon inside of a
ribosome, it brings the amino acid
into place so that it can join with
other amino acids to build the
polypeptide chain.
Check your work from the last slide
using the t-RNAs to the right.
Met
Leu
AUG
Ala
CUU
GCA
Tyr
Asp
Stop codons:
UAA
UAG
UGA
UAU
GAC
Translation happens in the cytoplasm.
First, the two r-RNA subunits join
together around the end of a mRNA strand to form a functional
ribosome.
Gly
t-RNA
site
t-RNA
site
m-RNA binding site
As the strand of m-RNA moves
through the ribosome, t-RNAs bind in
the other two binding sites if their
anticodons match the m-RNA codons.
Translation continues in the cytoplasm.
The t-RNAs bring amino acids into
position so that they can bond with
each other, lengthening the amino
acid chain.
Gly
Ala
Ser
Trp
t-RNA
site
t-RNA
site
m-RNA binding site
As the amino acid is attached, the
ribosome moves over one codon, the
leading t-RNA is released to get
another amino acid, and another tRNA moves into the free site.
Translation concludes.
This movement of the
ribosome, attachment of
t-RNAs, and lengthening
of the amino acid chain
continues until a stop
codon is reached.
The stop anticodon has no amino acid bound to
it. As it moves through the ribosome, the t-RNAs
detach from the ribosome and the polypeptide
chain bends and folds into a 3-D shape
depending on its amino acid sequence.
Leu
Ile
Tyr
t-RNA
site
t-RNA
site
m-RNA binding site