Download RNA and Protein Synthesis

RNA and Protein Synthesis
Chapter 8
Sections 4 - 7
3 main differences between RNA and DNA:
1. Sugar in RNA is ribose; Sugar in DNA is deoxyribose
2. RNA is single stranded; DNA is double stranded
3. RNA contains uracil in place of thymine
3 Types of RNA
1. Messenger RNA (mRNA)- disposable copy of DNA to carry instructions to
assemble proteins at the ribosome
2. Ribosomal RNA (rRNA)- RNA inside the ribosome to make the ribosome exist
3. Transfer RNA (tRNA)- transfers amino acids in the cytoplasm to the growing
polypeptide chain in the ribosomes to construct proteins
Protein Synthesis is the process of converting mRNA into a
protein. It is the process of how proteins are made.
Protein Synthesis has 2 steps:
1. Transcription
-takes place in nucleus
2. Translation
-takes place in ribosome
Transcription converts a DNA gene into a single-stranded mRNA molecule.
In other words, Transcription is a process that copies DNA to
make a strand of mRNA.
Takes place in Nucleus.
Transcription begins at specific locations on DNA called promoters.
Enzyme called RNA Polymerase separates DNA strand by breaking H bonds between
nucleotide pairs. Separated strands of DNA are used as templates to assemble strand
of mRNA.
RNA Polymerase
start site
nucleotides
DNA Templates
Nucleotides pair with one template strand of DNA. RNA polymerase bonds the
nucleotides back together. The DNA helix winds again as the gene is transcribed
mRNA
DNA
RNA Polymerase
Nucleotides
RNA polymerase moves
along the DNA
The mRNA strand detaches from the DNA once the gene is transcribed. It will leave
the nucleus to find a ribosome floating in the cytoplasm or find a ribosome on the
Rough ER. This event marks the end of Transcription and the beginning of
Translation.
mRNA
Transcription Overview
The transcription process is similar to replication.
• Transcription and replication both involve complex
enzymes and complementary base pairing.
• The two processes have different end results.
–
–
Replication copies
all the DNA;
transcription copies
a gene.
Replication makes
one copy;
transcription can
make many copies.
one
gene
growing RNA strands
DNA
Translation converts mRNA into a protein.
This process involves translating the language of nucleic acids (base sequences)
into a language of proteins (amino acids).
A gene carries a code to make one protein.
A gene can be anywhere from 300 to 3000 base pairs long.
Code written in language with only 4 “letters”: A, C, G, U
Code read 3 letters at a time (each 3 letter “word” known as a codon) to make an
amino acid.
codon for
methionine (Met)
***Amino acids are the building
blocks of proteins.
codon for
leucine (Leu)
Steps to Translation:
1. mRNA leaves nucleus and attaches to ribosome.
2. Translation begins with tRNA in ribosome binding to AUG, the start
codon, which signals the ribosome to assemble amino acids in a
polypeptide chain to create a protein.
3. Each tRNA has an anticodon whose bases are
complementary to the codon on mRNA. tRNA brings amino
acids to ribosomes and build a polypeptide chain, which creates
a protein.
anticodon
codon
4. Ribosome moves along mRNA, and bind new tRNA
molecules to mRNA codons and link amino acids together
to create a polypeptide chain.
5. The now empty tRNA molecule exits the ribosome.
6. A complementary tRNA molecule binds to the next exposed codon.
7. The polypeptide chain grows until tRNA reaches 1 of the 3 stop codons
(UGA, UAA, UAG). At the end of translation, the protein is released,
folded into a 3D structure, transported in a vesicle to the Golgi Apparatus to
be modified and “shipped” to its final destination within the cell or outside of
the cell.
Protein molecule
stop codon
The Genetic Code is the language of mRNA. tRNA translates this 4 letter
language into amino acids, so a protein can be created. You read 3 letters at time.
Example: AUG CCC GGG AUU UGA translates into the following amino acid
polypeptide chain: Methionine-Proline-Glycine-Isoleucine-STOP
STOP is not an amino acid. It simply tells the tRNA to terminate the translation
process and to not add anymore amino acids to the polypeptide chain.
•
The central dogma includes three processes:
1. Replication
2. Transcription
3. Translation
replication
transcription
• RNA is a link
between DNA
and proteins.
translation
Mutation- a change in an organism’s DNA that affects genetic information and may
or may not affect phenotype (physical appearance).
Gene Mutations result from changes in a single gene. Many kinds of mutations can
occur, especially during replication.
•Some gene mutations change phenotype.
–A mutation may cause a premature stop codon.
–A mutation may change protein shape or the active site.
–A mutation may change gene regulation.
Chromosomal Mutations- involves changes in the number and structure of
chromosomes. Chromosomal mutations usually occur during crossing over and may
affect many genes. Chromosomal mutations tend to have a big effect.
There are 7 main types of mutations:
1. Point Mutation (aka Substitution)
2. Frameshift Mutation
3. Deletion
4. Duplication
5. Insertion
6. Inversion
7. Translocation
1.
Point mutations (aka Substitutions)- occur at a single point in DNA
sequence and generally change one amino acid in the polypeptide chain.
Example: Sickle Cell Anemia
Sickle cell anemia is a disease in which red blood cells form an
abnormal crescent shape. (Red blood cells are normally shaped
like a disc.) Sickle-shaped cells deliver less oxygen to the
body's tissues. They also can clog more easily in small blood
vessels, and break into pieces that disrupt blood flow. People
with sickle-cell anemia have a survival advantage. Sickleshaped red blood cells are resistant to the infectious parasite
that causes malaria, and people with this trait were more likely
to survive malaria epidemics.
Point Mutation/Substition
A point mutation is a simple change in one base of the gene sequence.
This is equivalent to changing one letter in a sentence, such as this
example, where we change the 'c' in cat to an 'h':
Original:
The fat cat ate the red rat.
Point Mutation: The fat hat ate the red rat.
2. Frameshift mutation- insertion or deletion of nucleotide. It causes big changes
because it can alter protein shape by making it unable to perform normal functions.
Example: Tay-Sachs Disease Children with Tay-Sachs, a progressive
neurodegenerative disease that attacks nerve cells, usually die before age 5.
Frame-shift mutation
In a frame shift mutation, one or more bases are inserted or
deleted, the equivalent of adding or removing letters in a
sentence. But because our cells read DNA in three letter
"words", adding or removing one letter changes each
subsequent word. This type of mutation can make the DNA
meaningless and often results in a shortened protein. An
example of a frame-shift mutation using our sample sentence
is when the 't' from cat is removed, but we keep the original
letter spacing:
Original:
The fat cat ate the red rat.
Frame Shift: The fat caa tet her edr at.
3. Deletion
Mutations that result in missing DNA are called deletions.
These can be small, such as the removal of just one "word," or
longer deletions that affect a large number of genes on the
chromosome. Deletions can also cause frameshift mutations.
In this example, the deletion eliminated the word cat.
 Original: The fat cat ate the red rat.
 Deletion: The fat ate the red rat.
 Example: Cystic Fibrosis
4. Duplication
Mutations that result in the addition of an extra copy of DNA
are called duplications. Duplications can also cause
frameshift mutations, and general result in a nonfunctional
protein.
 Original: The fat cat ate the red rat.
 Insertion: The fat cat cat ate the red rat.
5. Insertion
Mutations that result in the addition of extra DNA are called
insertions. Insertions can also cause frameshift mutations,
and general result in a nonfunctional protein.
 Original: The fat cat ate the red rat.
 Insertion: The fat cat dog ate the red rat.
6. Inversion
In an inversion mutation, an entire section of DNA is
reversed. A small inversion may involve only a few bases
within a gene, while longer inversions involve large regions
of a chromosome containing several genes.
 Original: The fat cat ate the red rat.
 Insertion: The fat tar der eht eta tac.
7. Translocation
Translocation results from the exchange of DNA
segments between nonhomologous chromosomes.
More example diagrams of mutations
• Some gene mutations do not affect phenotype.
– A mutation may be silent.
– A mutation may occur in a noncoding region.
– A mutation may not affect protein folding or the active site.
•
•
•
•
Mutations in body cells do not affect offspring.
Mutations in sex cells can be harmful or beneficial to offspring.
Natural selection often removes mutant alleles from a population when they are less
adaptive.
Mutations may have led to drastic and quick evolutionary changes
Mutations can be caused by several factors.
• Replication errors can cause
mutations.
• Mutagens, such as UV ray and
chemicals, can cause mutations.
• Some cancer drugs use
mutagenic properties to kill
cancer cells.