Transcript Gene Expression

Gene Expression
Transcription and translation
Types of RNA
• Messenger RNA (mRNA) carries copies of
instructions for assembling amino acids into
proteins.
Types of RNA
• Ribosomes are made
up of proteins and
ribosomal RNA
(rRNA).
Types of RNA
• During protein
construction, transfer
RNA (tRNA) transfers
each amino acid to the
ribosome.
Gene Expression: How Proteins are
Made
 Directing the production of PROTEINS is the job of
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DNA.
Your genes contain codes for proteins, which are
responsible for your “TRAITS”.
Some protein products can be seen directly.
MELANIN, the protein that gives your skin color is an
example.
Other proteins regulate your DEVELOPMENT or
body processes.
The process of protein synthesis occurs in TWO
stages, and at two different LOCATIONS in the cell.
Important things to remember:
 DNA holds the code for all proteins made by
a living thing.
 DNA is found in the NUCLEUS of a cell and
CANNOT leave the nucleus.
 Proteins are constructed by organelles
called RIBOSOMES, which are in the
CYTOPLASM of a cell.
 Proteins are made of hundreds or thousands
of AMINO ACIDS bonded together.
Step One: Transcription
 Occurs in the NUCLEUS of the cell.
 The information in a gene is copied into
MESSENGER RNA (mRNA). This copy will be able
to leave the nucleus, the DNA cannot.
 mRNA is the equivalent of a PHOTOCOPY of DNA. It
is only single stranded, but carries all of the
information in the DNA code for a particular gene.
 There are no THYMINES in RNA. All ADENINES in
DNA correspond to a base called URACIL (U) in
RNA.
Trascription
RNA leaving the nucleus
The Genetic Code
 The Genetic Code
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The genetic code is the “language” of mRNA
instructions.
The code is written using four “letters” (the
bases: A, U, C, and G).
The Genetic Code
 A codon consists of three consecutive
nucleotides on mRNA that specify a particular
amino acid.
The Genetic Code
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Each codon specifies a particular amino acid that is to
be placed on the polypeptide chain.
Some amino acids can be specified by more than one
codon.
The Genetic Code
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There is one codon AUG that can either specify the
amino acid methionine or serve as a “start” codon for
protein synthesis.
There are three “stop” codons that do not code for any
amino acid. These “stop” codons signify the end of a
polypeptide.
Step Two: Translation
 The mRNA leaves the nucleus through a NUCLEAR
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PORE and enters the cytoplasm.
The RIBOSOMES of a cell are the organelles that
actually assemble the proteins and they are in the
cytoplasm.
Ribosomes attach to the mRNA and read it 3 bases
at a time. A unit of 3 bases is called a CODON.
Each codon codes for one AMINO ACID (the building
blocks of proteins).
The codons on the mRNA base pair with ANTICODONS on tRNA (transfer RNA) molecules.
Step Two: Translation continued
 The tRNAs bring amino acids to the ribosome one at
a time.
 The message in a gene begins at an INITIATOR or
“START” codon on the mRNA. This codon is AUG. If
you are asked to translate a gene and the mRNA
doesn’t begin with AUG, you must search and begin
the process at the location of the AUG codon.
 The message in a gene ends at a TERMINATION or
“STOP” codon. There are THREE different stop
codons.
 The stop codon signals the RIBOSOME to
disassemble. The mRNA and the finished
POLYPEPTIDE are released.
Translation:
Protein Synthesis Odds and Ends:
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A ribosome is made of rRNA and PROTEINS.
A ribosome is made of TWO subunits, the LARGE
ribosomal subunit and the SMALL ribosomal subunit.
MULTIPLE ribosomes may read a single piece of
mRNA at the same time.
TWO tRNA molecules can be in the ribosome at the
same time.
The bonds that form between amino acids are called
PEPTIDE BONDS formed by dehydration synthesis.
The start codon, AUG codes for the amino acid
METHIONINE. In most cases, finished proteins do not
begin with methionine because it is removed during
processing.
RNA Editing
 RNA Editing
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The DNA of eukaryotic genes contains
sequences of nucleotides, called introns, that are
not involved in coding for proteins.
The DNA sequences that code for proteins are
called exons.
When RNA molecules are formed, introns and
exons are copied from DNA.
RNA Editing
Exon Intron
 The introns are
cut out of RNA
molecules.
DNA
Pre-mRNA
 The exons are the
spliced together
to form mRNA.
mRNA
Cap
Tail
Chromosome 7 activity
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Task: transcribe and translate a gene
Answer = sequence of amino acids
RNA polymerase reads from 3’5’
Start at 3’ at top of page
Code twists along with helix
Promoter/Initiator region on DNA = ATATTAG
Promoter/Initiator gets transcribed
Terminator region on DNA = CCCC
Intron on mRNA = UAGC
Polyribosome
Genes on a chromosome
Genes on a chromosome
Warm up:
 If a finished protein contains 100 amino acids
and begins with methionine, what is the
MINIMUM number of codons used to code for
this protein?
 How many peptide bonds are needed to hold
together the protein described above?
 How many bases are in the mRNA sequence
for this protein?
 How many base pairs of DNA long is the
gene for this protein?
Warm Up Answers
 If a finished protein contains 100 amino acids and
begins with methionine, what is the MINIMUM
number of codons used to code for this protein?
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101 (1 for each amino acid and a STOP codon)
 How many peptide bonds are needed to hold
together the protein described above?
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99
 How many bases are in the mRNA sequence for this
protein?
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303 (101 codons x 3 bases per codon)
 How many base pairs of DNA long is the gene for this
protein?
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303
Warm Up Two:
 If a finished protein contains 100 amino acids
and does not begin with methionine, what is
the MINIMUM number of codons used to
code for this protein?
 How many peptide bonds are needed to hold
together the protein described above?
 How many bases are in the mRNA sequence
for this protein?
 How many base pairs of DNA long is the
gene for this protein?
Warm Up Two:
 If a finished protein contains 100 amino acids and does not
begin with methionine, what is the MINIMUM number of codons
used to code for this protein?
 102 (start codon + 1 per aa + stop codon)
 How many peptide bonds are needed to hold together the
protein described above?
 99
 How many bases are in the mRNA sequence for this protein?
 306
 How many base pairs of DNA long is the gene for this protein?
 306