Chapter 12-3: RNA and Protein Synthesis

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Chapter 12-3RNA and Protein Synthesis
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What is a gene?

• A gene is a set of DNA instructions that control
  the synthesis of proteins within the cell.
• This process, called protein synthesis, involves
  2 steps transcription and translation.

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How does a gene work?

• DNA cannot leave the nucleus, so a copy is made
  in the form of a similar nucleic acid called RNA
  (ribonucleic acid) during transcription. After
  DNA is used to make RNA, the RNA is then used to
  make proteins during translation by ribosomes in
  the cytoplasm.

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How is DNA different from RNA?

• RNA is a nucleic acid similar to DNA with 3
  differences
• RNA contains the sugar ribose and DNA contains
  the sugar deoxyribose
• RNA is single-stranded, DNA is double-stranded

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How is DNA different from RNA?

1. RNA contains uracil instead of thymine. Uracil,
   like thymine, is a pyrimidine (contains one
   ring).

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Three types of RNA

• messenger RNA (mRNA)
• carries a copy of the information in DNA.
• This type of RNA acts as a messenger bringing
  the information in the DNA to the ribosome.

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• ribosomal RNA (rRNA)
• this RNA is found with ribosomes.
• Recall that ribosomes synthesize proteins.
• Ribosomes are made of both proteins and rRNA.

2 subunits
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Label this figure on your notes! Transfer RNA

• 3. transfer RNA (tRNA)
• this RNA brings amino acids to the ribosome to
  be added to the polypeptide chain that is being
  made at the ribosome.
• There is one tRNA for each of the 20 amino acids.

Amino acid
tRNA
anticodon
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Central Dogma of Molecular Biology

• See board

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Transcription

1. Transcription is the process in which the
   nucleotide sequence of DNA is copied, or
   transcribed into a complementary nucleotide
   sequence of RNA.
2. DNA is like the master plan and RNA is like the
   blueprint. The master plan stays in the
   nucleus (the office) and the blueprint can be
   taken to the ribosome (construction site).
3. Because DNA is in the nucleus, transcription
   occurs in the nucleus of the cell.
4. When transcription is finished, the RNA moves to
   the cytoplasm. DNA cannot leave the nucleus!!

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Transcription

• 5. The enzyme RNA polymerase is used to make
  RNA. RNA polymerase
• A. binds to the DNA,
• B. separates the DNA, and
• C. uses only one strand of DNA as a template to
  make a complementary RNA strand. RNA Polymerase
  reads 3 ? 5

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Transcription
D. Once RNA is made, the RNA detaches from the
template strand, leaves the nucleus through
nuclear pores, and enters the cytoplasm of the
cell
RNApolymerase
DNA
RNA
How does RNA look different than DNA? Do RNA and
DNA leave the nucleus?
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Video Clip Transcription
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Translation

1. RNA is used to make protein in a process called
   translation.
2. Remember that proteins, a.k.a. polypeptides, are
   macromolecules made of monomers called amino
   acids.

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• The Genetic Code
• In RNA, the nucleotides are read in words made
  of 3 nucleotide letters. Each word is called
  a codon and contains the genetic code for one
  amino acid.

Move from the inside out!
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• The first word or start codon is always the
  same for every protein. It is always AUG. Using
  the genetic code wheel you can figure out which
  amino acid a codon represents. What amino acid
  is associated with the codon AUG?
• There are a total of 20 different amino acids
  that can be arranged in different ways to make
  different proteins.

Methionine
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Activity 1
Transcription in nucleus
Translation in cytoplasm
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Translation

1. During translation, ribosomes decode the mRNA
   message (made of nucleotides) to make polypeptide
   chains (made of amino acids). Ribosomes read 5
   ? 3 on the mRNA.
2. Before translation can occur, mRNA must be made
   from DNA in transcription. Transcription occurs
   in the nucleus, then the mRNA travels out of the
   nucleus into the cytoplasm.
3. Translation is done by ribosomes in the
   cytoplasm.

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• The steps in translation include
• 1. Two subunits of the ribosome attach to the
  mRNA.

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1. Ribosomes read 5 ? 3. As each codon of the
   mRNA moves through the ribosome, the correct
   amino acid is brought to the ribosome by tRNA.
2. Each tRNA molecule has a group of three
   nucleotides called the anticodon. These three
   nucleotides pair with the nucleotides in the
   codon. The tRNA molecule also has an amino acid
   attached.

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• 4. Peptide bonds form between amino acids to
  form the polypeptide chain in a process called
  elongation because it makes the polypeptide chain
  longer.

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• This forms a peptide bond between the amino
  acids. This is why proteins are called
  polypeptides.
• The empty tRNA molecule exits the mRNA and is
  recycled by the cell and can bind another amino
  acid.

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1. The ribosome continues to match the codons in the
   mRNA with anticodons in tRNA until it reads a
   codon in the mRNA that says stop. A tRNA for
   stop does not carry an amino acid. No peptide
   bond will form, so the ribosome releases the mRNA
   and the protein.

6. Polypeptide will start to coil and bend,
forming the 3-D shape of proteins (recall the 4
level of protein structure)
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Video Clip Translation
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Why are proteins important?

• Each protein has a specific function within
  living cells. Some functions of proteins are
• acting as enzymes, to speed up and regulate
  chemical reactions.
• making pigments that determine flower color in
  plants
• determining your blood type.
• regulating cell growth and development.

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Where in the cell does transcription
occur? -Nucleus
Where in the cell does translation occur?
-Cytoplasm
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Chapter 12-4Mutations
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• Mutations are changes in the genetic code. They
  come from mistakes that cells have made in
  copying their own DNA. If a cell has a mutation,
  all of its daughter cells will have that same
  mutation.

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• Kinds of mutations
• Mutations come in many shapes and sizes.
• Mutations that produce a change in a single gene
  are called gene mutations.
• Other mutations produce changes in whole
  chromosomes (and affect many genes). These
  mutations are called chromosomal mutations.

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Gene mutations affect one gene

• Point mutations involve changes in one or a few
  nucleotides at one point in the DNA sequence.
• Substitutions where one base is changed to
  another. These usually affect only one amino
  acid.

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Original DNA The fat cat ate the rat. Mutant
DNA The fat hat ate the rat. (substitution)
Substitution
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• Frameshift mutations (Insertions or Deletions)
  an extra base is added or removed. These
  usually affect a large part of the protein.
  Remember, bases are read in groups of three, but
  if one base is added or removed, this shifts the
  reading frame of the genetic code and can
  change all amino acids after the site of the
  mutation

Original DNA The fat cat ate the rat. Mutant
DNA The fat cat tat eth era t. (insertion)
Insertion
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• Frameshift mutations (Insertions or Deletions)
  an extra base is added or removed. These
  usually affect a large part of the protein.
  Remember, bases are read in groups of three, but
  if one base is added or removed, this shifts the
  reading frame of the genetic code and can
  change all amino acids after the site of the
  mutation

Deletion
TAG CAT GGA AT
AUC GUA CCU UT
Val
Pro
Ile

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Video Clip Point Mutations
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1. Chromosomal mutations involve changes in the
   number or structure of the chromosomes.

• Can change the locations of genes on chromosomes
• Inversion reverses the direction of parts of
  the chromosomes
• Translocation part of one chromosome breaks off
  and attaches to another.

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Video Clip Translocation and Inversion
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• Can change the number of copies of some genes
• Deletion a part of the chromosome is lost
• Duplication there is an extra copy of part of
  the chromosome

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Video Clip Duplication and Deletions
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  Effects of Mutations

1. Neutral No effect on protein function
2. Harmful Cause genetic diseases
3. Beneficial Plants with extra sets of
   chromosomes are larger and stronger
4. Any new trait in a population, good or bad, is a
   result of a mutation.

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Leu
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