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Protein Synthesis
Chapter 12 p. 300
Transcription and Translation
How does DNA do it??
• https://www.youtube.com/watch?v=zwibgNG
e4aY
Protein Synthesis
In molecular terms, genes are coded DNA
instructions that control the production of
proteins in the cell.
In order for the cell to synthesize proteins, it
must get instructions from DNA, but the DNA
never leaves the nucleus and proteins are
synthesized by ribosomes on the RER or in the
cytoplasm.
RNA to the rescue…
Protein synthesis takes place in two steps:
Transcription
- occurs in the nucleus
- mRNA is made from a strand of DNA
and
Translation
- occurs in the cytoplasm
- a protein is put together (synthesized) by a
ribosome using a strand of mRNA
Three types of RNA take part in the synthesizing
of proteins from the code found on a gene.
• mRNA
– messenger
• rRNA
– ribosomal
• tRNA
– transfer
A dazzling display of choreography:
• http://www.youtube.com/watch?v=41_Ne5m
S2ls
Transcription
• Occurs in the nucleus
• Is the process by which part of the DNA
sequence is copied into a complementary
sequence of RNA
During transcription,
• An enzyme called RNA polymerase
– binds to DNA and separates the strands
– then uses one strand of DNA, the template
strand, as a template  from which nucleotides
are assembled to create a strand of RNA.
• RNA polymerase will only bind to a strand of DNA
in regions called promoters.
• In genetics, a promoter is a region of DNA where
transcription of a gene is initiated (started).
Promoters are located near the genes they
transcribe, on the same strand and upstream on
the DNA (towards the 3’region of the template
strand.) Promoters can be about 100–1000 base
pairs long.
• A gene on DNA includes introns and exons.
When the RNA is copied it also includes
introns and exons.
• Introns are those sequences on a gene which
are not used in the synthesis of proteins.
Exons are those sequences of the gene which
are used in the synthesis of proteins.
• After the RNA is synthesized and while it is still
in the nucleus, the introns are cut out and the
exons are spliced together. This makes mRNA.
Introns and Exons
• What actually does the cutting and splicing
varies depending on the gene and our
knowledge of this process is far from
complete.
• An excerpt from Wikipedia:
At least four distinct classes of introns have been
identified.[1]
• Introns in nuclear protein-coding genes that are
removed by spliceosomes
• Introns in nuclear and archaeal transfer RNA genes that
are removed by proteins (tRNA splicing enzymes)
• Self-splicing group I introns that are removed by RNA
catalysis.
• Self-splicing group II introns that are removed by RNA
catalysis
mRNA
mRNA
The “language” of mRNA is called the genetic
code.
Remember that mRNA contains 4 bases: A, U,
C, and G.
Transcription (review) and Translation
Introduction
• http://www.youtube.com/watch?v=LY0hZLDO
b00
Translation
• Is the process by which the code on mRNA is
translated into a protein (a polypeptide chain).
• rRNA reads the code in groups of three bases.
• Each group of 3 letters is a codon.
• Each codon represents a specific amino acid (AA).
Amino Acids
Remember that proteins are made of
amino acids joined by peptide bonds.
• Translation begins when an rRNA attaches to
mRNA in the cytoplasm.
http://www.youtube.com/watch?v=zb6r1MMTkc&feature=related
Translation
Translation
• Initiator (start) codons: The point along a
strand of mRNA where a rRNA molecule
begins to translate a sequence of mRNA into
amino acids. AUG
• Terminator or stop codons: The point where
rRNA stops translating the sequence and
releases the amino acid chain. UAA UAG
UGA
• The proteins produced may vary from 2
codons in length to several thousand codons
in length.
– Ex. 60 amino acids
• How many nucleotides?
• How long is the gene In codons?
• Once the mRNA leaves the nucleus to go to a
ribosome, the next step takes place.
• Other genes in the DNA produce a transfer
RNA (tRNA), shaped loosely like a cloverleaf.
• One end of tRNA is attached to a specific AA
(1 of 20) and the other end has a set of 3
exposed bases called an anticodon.
Ex. Codon from mRNA = UCG
Anticodon from tRNA = AGC
• The rRNA is the part of the ribosome that
reads the code on the mRNA and helps the
amino acids to bond.
• rRNA pulls the mRNA through the ribosome
and directs a tRNA (whose anti-codon is
complimentary to the mRNA codon) into the
codon-anticodon slot.
• Once the codon and anti-codon are
connected, the tRNA releases its amino acid
which is added to the chain of amino acids
growing from the ribosome.
• The amino acids are joined by peptide bonds.
As each is added, a water molecule is
released. (Dehydration hydrolysis)
• The chain will continue to grow until the rRNA
reads the mRNA stop codon. (UAG, UAA or
UGA)
• Once the tRNA has given up its amino acid, it
can get another one.
• The process of decoding an mRNA message to
build a polypeptide chain (protein) is called
translation.
• Many ribosomes can be working on a single
strand of mRNA.
Mutations (page 307)
• Mutations are changes in the genetic
material.
Mutations that produce changes in a gene are
called genetic mutations.
Mutations that produce changes in the whole
chromosome or a portion of the chromosome
are called chromosomal mutations.
Gene Mutations:
1. Point mutations:
– Can include
– substitution (one base changed to another)
which usually only affects the codon it is
found in, therefore, only a single amino
acid. This can still have devastating effects.
EX. THE FAT CAT ATE THE RAT
THE TAT CAT ATE THE RAT
Gene mutations cont’d
2. Frameshift mutations include:
deletions and insertions which can be quite
dramatic because the bases are read in groups
of 3 and an insertion or deletion will shift
everything.
Deletion:
THE FAT CAT ATE THE RAT
THE ATC ATA TET HER AT
Insertion:
THE FAT CAT ATE THE RAT
THE FFA TCA TAT ETH ETA T
• By altering the code, every amino acid
following the insertion or deletion could be
different that what it was supposed to be.
• Frameshift mutations can, therefore, alter a
protein so much that it is unable to do its job.
Chromosomal Mutations
• Involve changes in the number or structure of chromosomes.
EX.
123-456
134-56
122-345-6
165-432
Normal
Deletion
Insertion
Inversion
123-789
78-9456
Translocation (Part of one
chromosome breaks off
and attaches to another.)
Significance of Mutations
• Most mutations are neutral (have little or no
effect).
• They can cause dramatic changes in the
structure of protein or gene activity which can
cause:

cause disruptions in normal biological
activities or genetic disorders or

be beneficial and account for evolution.
(Organisms in changing environments/survival
of the fittest)
• Mutations in the cells that make gametes can
be passed along to offspring.
• A condition in which an organism has an extra
set of chromosomes is called polyploidy,
caused when a complete set of chromosomes
fails to separate. Polyploidy plants are often
larger and stronger than diploid plants so are
bred that way. (bananas, citrus fruit)
• http://www.youtube.com/watch?v=LuJuJC3ZH
wA
Homework
• Page 305 Questions 1 – 5
• Page 308 Questions 1 -5