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/watch?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
Translation1. ribosome attaches to mRNA2. 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
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