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RNA and Protein Synthesis

RNA structure, its types, transcription, its editing process, the genetic code, and translation
by

Miss Schwinge

on 10 October 2014

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Transcript of RNA and Protein Synthesis

and Protein Synthesis
RNA
The double helix structure of DNA explains how it can be copied, but it does not explain how a gene works.
Genes
The structure of
RNA
RNA molecules have
many functions,
but in the majority of cells
most RNA molecules
are involved in just
one job: protein synthesis.
Types of RNA
Genes are molecular units of heredity. They are coded DNA instructions that control the production of proteins within the cell, and decide what the organism is like. Humans have roughly 22,000 genes.
The first step in decoding these genetic messages is to copy part of the nucleotide sequence from DNA into RNA, or ribonucleic acid.
These RNA molecules contain coded information for making proteins
RNA,
like DNA,
consists of a long chain of nucleotides,
and is made up of the
same things as DNA
(a 5-carbon sugar, a phosphate group and a nitrogenous base)
except

for one small difference:
the sugar group in RNA is ribose.
RNA also differs from DNA
in the fact that it is
single stranded, and has the base uracil instead of thymine
(therefore, in RNA
A bonds with U
).
RNA is like a
disposable copy
of a
segment of DNA
In many cases, an
RNA molecule is a working copy of a single gene.
The ability to
copy a single DNA sequence into RNA
makes it possible for a
single gene to produce hundreds or even thousands of RNA molecules
There are
three main types
of RNA:
1.) messenger RNA (mRNA)
2.) ribosomal RNA (rRNA)
3.) transfer RNA (tRNA)
The job of
messenger RNA (mRNA)
is to
carry copies of instructions
for the
assembly of amino acids into proteins from DNA
to the rest of the cell
The job of
transfer RNA (tRNA)
is to
transfer, or move, amino acids to ribosomes
during
protein synthesis
The job of
ribosomal RNA (rRNA)
is to
make up the major part of ribosomes. Proteins are assembled on ribosomes,
and ribosomes are made of
several dozen proteins as well as rRNA.
Transcription
RNA molecules
are produced by
copying part of the nucleotide sequence of DNA
into a
complementary sequence in RNA
.
This process is called
transcription.
Transcription
requires an
enzyme
known as
RNA polymerase
, which is
very similar
to the DNA polymerase in
DNA replication.
During
transcription,
RNA polymerase
binds to DNA and separates the DNA strands.
But how does RNA polymerase "know" where to start and stop making an RNA copy of DNA?

RNA polymerase doesn't bind to DNA just anywhere
, it only
binds to regions of DNA
known as
promoters
, which have
specific base sequences.
Promoters
are
signals in DNA
that tell the enzyme
where to bind
to make RNA.
Similar signals
in DNA cause transcription to
stop
when the new RNA molecule is
completed.
RNA Editing
Our DNA contains
sequences of nucleotides
called
introns
that are
not involved in coding for proteins.
The
DNA sequences
that
code for proteins
are called
exons
because they are
"expressed"
in the synthesis of proteins.
When
RNA molecules
are
formed,
both the
introns
and
exons
are
copied
from the
DNA.
However, the
introns are cut out of RNA
molecules while they are still in the
nucleus.
The remaining
exons
are then
spliced back together
to form the
final strand of mRNA.
The Genetic Code
Proteins

are made by
joining amino acids
into

long chains
called
polypeptides.
Each polypeptide contains a
combination
of any or all of the
20 different amino acids.
The
properties
of proteins are determined by the
order
in which
different amino acids
are
joined
together to
produce polypeptides.
But how can
nitrogenous bases
(just four letters) make up the
amino acids
that make up the
polypeptides
which make up the
proteins?
How can a
code
w
ith just
four letters

carry instructions for
20
different amino acids?
The
genetic code
is read
three letters
at a time, so that each
"word"
of the coded message is
three bases long
. Each three-letter
"word" in mRNA
is known as a
codon.
A
codon
consists of
three
consecutive nucleotides that specify a
single amino acid
that is to be
added
to the
polypeptide.
For example, consider the following RNA sequence:
UCGCACGGU

It would be read three bases at a time:
UCG-CAC-GGU

And these three codons represent the different amino acids:
UCG- CAC- GGU
Serine-Histidine-Glycine
Because there are
four different bases,
there are
64
possible three-based codons (4x4x4 =64)
This wheel shows all 64 possible codons of the genetic code.

Some amino acids
can be specified by
more than one codon.
You start at the center of the circle and move outwards to find the amino acid.
There is also one codon,
AUG
, that can either specify for the
amino acid methionine
or serve as the

"start" codon
for protein synthesis.
There are also
three "stop" codons
that do
not
code for
any amino acid. Stop codons
act like the
period

at the end of a sentence; they signify the
end of the polypeptide
.
Translation
The
sequence
of nucleotide
bases
in an
mRNA molecule
serves as
instructions for the order
in which
amino acids
should be
joined
together to produce a
polypeptide
. But we then need something to
"read"

these instructions and
put them to use
, and that is the job of a tiny factory called the
ribosome.
The
decoding
of an
mRNA message
into a polypeptide chain
(protein)
is known as
translation.
Translation takes place on
ribosomes.
During
translation
,
the cell uses
information from mRNA
to produce
proteins.
Before
translation occurs,
mRNA is transcribed from DNA
in the
nucleus
and
released
into the
cytoplasm.
Translation
begins

when an
mRNA molecule
in the cytoplasm
attaches
to a
ribosome
. As each
codon

of the
mRNA molecule
moves through the
ribosome,
the
proper amino acid
is brought
into
the ribosome by
tRNA
. In the ribosome, the
amino acid

is
transferred

to the growing
polypeptide chain.
In addition to an amino acid, each tRNA molecule has
three unpaired bases.
These bases, called the
anticodon
,
are
complementary

to one
mRNA codon.

Example: The
anticodon
for
AUG
would be
UAC
Like an assembly line worker who attaches one part to another, the
ribosome forms a peptide bond
between the
first two amino acids
as well as
breaks the bonds
that had
held

the first
tRNA
to its
amino acid,
which
releases
the
tRNA
molecule.
The
polypeptide chain
continues to grow until the ribosome reaches a
stop codon

on the mRNA molecule. When the ribosome reaches the stop codon, it
releases
the
newly formed polypeptide
and the
mRNA
molecule,
completing
the process of
translation.
Mutations
Now and then cells make
mistakes

in copying their own DNA
. For instance, they can
insert
an
incorrect base
or even
skip a base
as the
new strand is put together
.
There are
many different types
of mutations.
Mutations
that
produce changes
in a
single gene
are known as
gene mutations.
Gene mutations
involving
changes
in
one or a few nucleotides
are known as
point mutations
because they occur at a
single point in the DNA sequence
.
Point mutations
include
substitutions,
in which
one base is changed to another
.
There are also
insertion and deletion mutations
, in which a
base is added or removed
from the
DNA sequence
.
Substitutions
usually affect
no more
than a
single amino acid
, but the effects of
insertions or deletions
can be
much more dramatic.
Remember that the
genetic code
is read in
three-base codons
. If a
nucleotide
is
added or deleted
, the bases are still
read in groups of three
, but now these groupings are
shifted
for every
codon that follows.
These are called
frameshift mutations
because they
shift
the
"reading frame"
of the
genetic message.
By shifting the reading frame,
frameshift mutations
may
change

every amino acid
that
follows the point of the mutation
. Frameshift mutations can
alter a protein so much
that it is
unable to perform its normal functions
.
There are also
mutations
that
produce changes
in whole
chromosomes
, such as their
number
or
structure
, known as
chromosomal mutations
. They may change the
locations of genes
on
chromosomes
, and may even
change the number of copies of some genes.
There are
four main types
of
chromosomal mutations:
1.) Deletion
2.) Duplication
3.) Inversion
4.) Translocation
Deletions
involve the
loss of all or part
of a
chromosome
, while
duplications produce extra copies
of parts of a chromosome.
Inversions reverse the direction
of parts of a chromosome, and
translocations
occur when
part of one chromosome breaks off and attaches to another.
Mutations
are the
cause
of
many genetic disorders
, including
cystic fibrosis,
and
harmful mutations
are also associated with many types of
cancer
. However,
many
, if not most,
mutations are neutral
, meaning that they have
little or no effect
on the
expression of genes
or the
function of the proteins
for which they code for.
However,
mutations
are also the
source of genetic variability

in a species,
and some of this variation may be
highly beneficial.
One beneficial mutation in particular produces
resistance to HIV
, the virus that causes AIDS.
As A Reminder:
DNA ---> DNA is called replication
DNA ---> mRNA is called Transcription
mRNA ---> proteins (with the help of tRNA and ribosomes) is called Translation
Gene Regulation
Specialization
of
cells
in multicellular organisms is usually due to
different patterns of gene expression
rather than to
differences of the genes themselves.
An
expressed gene
is a gene that is
transcribed into RNA.
•Although human cells can have around
22,000 genes
, not all of them are
transcribed and translated all of the time
. Cells are able to
regulate (control)
which genes are
expressed
depending on the cell’s
needs.
In bacteria, a
group of genes
that
operate together
is known as an
operon.
The
on-off switch
that
turns on (transcribes and translates)
the
needed genes
is called the
operator
. It is a
piece of DNA
that overlaps the
promoter site.
Because of its position, it is able to
control RNA polymerase’s access
to the
genes.
When the
operon
is
“off,”
a
repressor protein

binds
to an
operator
and
physically blocks RNA polymerase
from binding to a
promoter site
and
stops the transcription
of the
genes
in the operon.
When the
operon
is
“on,”
the
repressor protein
has a
changed shape
and
falls off of the operator.
Now
RNA polymerase
is
not blocked
and the cell can
begin transcription.
By
producing the enzymes
only when the nutrient is
available,
the cell
saves energy.
Eye color is determined by the amount of melanin present in your irises. Blue eyes are due to a lack of melanin, while brown eyes indicate melanin-rich irises.
Heterochromia iridium (two different-colored eyes within a single individual)





and heterochromia iridis (a variety of color within a single iris) are relatively rare in humans and result from increased or decreased pigmentation of the iris due to a mutation in one of the genes that codes for eye color.
The assembly of amino acids into proteins is controlled by RNA
RNA polymerase
then uses
one strand of DNA
as a
template
to
assemble nucleotides
into
a strand of RNA.
Like a writer's first draft, many RNA molecules require
a bit of editing

before they are ready to go into action.
Remember:
RNA molecules are produced by copying DNA.
Each
tRNA
molecule carries only
one kind of amino acid
(for example, methionine).
These
mistakes
are called
mutations. Mutations
are
changes
in the
genetic material.
Mutations
that cause
dramatic changes
in
protein structure or gene activity
are often
harmful
, producing
defective proteins
that
disrupt normal
biological activities.
Full transcript