Download gene expression - Aurora City Schools

Notes: Chapter 13: RNA &
Protein Synthesis
• 2 steps:
– Transcription (DNA
is made into mRNA
in the nucleus)
– Translation (mRNA
is made into
proteins by
ribosomes in the
cytoplasm or ER)
RNA Structure
• Similar to DNA, but with some
important differences:
– Single strand (DNA double)
– Ribose is the sugar (not deoxyribose like
DNA)
– Uracil matches adenine (instead of
thymine like DNA)
– Shorter than DNA
RNA Functions
• Three different types
of RNA:
– mRNA (messenger)
used as template to
make proteins
– rRNA (ribosomal)
makes up ribosomes
– tRNA (transfer)
matches amino acids
to mRNA to help
make proteins
Transcription
• transcribe (to copy)
• happens in the nucleus
• DNA’s code is copied
onto RNA
– 1. RNA polymerase
binds to beginning of a
gene (promoter) and
unwinds DNA
– 2. Complementary
bases copied from 1
side of DNA
– 3. Termination (end)
signal reached,
mRNA and DNA
free
• http://www.youtube.com/watc
h?v=ztPkv7wc3yU
Translation
• RNA’s “language” is translated into amino
acids (which will become a protein)
• RNA’s language is set of three nucleotides
called a codon…3 nitrogen bases in a row
• Codons match with specific amino acids to
make polypeptide chain (which will be
modified to make a protein)
• 20 amino acids all together
• More than 20 codons, so some amino
acids have more than one codon.
There is also 1 start codon and 3 stop
codons.
Translation
Translation
1. ribosome attaches to mRNA
2. tRNA with amino acid matches mRNA
codon (area on tRNA that matches called
an anticodon). This process is called
initiation. 2 tRNAs can fit at one time.
3. ribosome moves down and matches next
codon.
4. Amino acids form peptide bond and
protein continues to grow, 1 amino acid at
a time. This process is called elongation.
5. ribosome reaches stop codon, mRNA,
tRNAs, protein and ribosome released.
This process is called termination.
The product is called a polypeptide. Modified in
ER or Golgi to make a protein.
Translation
• Each mRNA can be used more than
once, by
more than 1 ribosome at a time, so many
proteins can be translated from 1 transcribed
piece mRNA.
• http://www.youtube.com/watch?v=zb6r1MMTkc
Gene expression
• The whole
process of
going from
DNA to RNA
to a protein
that runs a
cell process
is called gene
expression
Gene expression
• After the polypeptide
chain is made, it
is often modified
in the ER or Golgi
Apparatus
• The finished product
is a protein and it
has a three
dimensional
shape that
decides its
function
Mutations
• Certain things can cause changes to the
DNA/RNA code. They are called
mutagens and cause mutations.
• e.g. UV light, smoking, X rays, exposure
to chemicals
• Not all are bad…some allow for
adaptation and evolution
Point mutations
• Occur to one base or a small number
of bases on DNA or RNA
• Most are harmless, as many codons
often code for the same amino acid.
• 3 kinds:
– Substitution: one base is used instead of
another
– Insertion*: one base is added
– Deletion*: one base is left out
– * lead to a frameshift mutation, where
everything is “off by one” base.
Point mutations
Chromosomal mutations
• Change to a gene, series of genes or
an entire chromosome
• These are often helpful in plants, e.g.
polyploidy…chromosomes do not
separate during meiosis. Leads to
seedless fruit that are bigger
• In animals, they are often harmful or
even lethal. You might be
researching one disorder in the
next unit on genetics…
Chromosomal mutations
• Deletion: one or more
genes is/are left out
when copied
• Duplication: one or more
genes is/are copied more
than once
• Inversion: one or more
genes is/are copied
backwards
• Translocation: one or more
genes is/are moved from
1 chromosome to another
• Nondisjunction (not shown):
homologs fail to separate
during meiosis
Transcription