The Central Dogma of Biology
Chapter 8 in your textbook

 Used to make proteins
DNA
Thymine
Double – stranded
1 type
Deoxyribose sugar
RNA
Uracil
Single
– stranded
3 types
Ribose sugar

Video clip – 2:52
1.
mRNA ( messenger ): copies the information from the DNA to carry it out of the nucleus to the ribosomes
2.
rRNA ( ribosomal ): used to make up most of the ribosomal subunits that decode the mRNA
3.
tRNA ( transfer ): carries amino acids to the ribosome to be linked together to form an amino acid chain (that will become a protein)

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 mRNA is linear (forms a line of bases) tRNA has a “ cloverleaf
” shape rRNA + protein make up the ribosome

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DNA carries the genetic code (genes)
The code is a triplet code – 3 nucleotides (grouped together as a codon ) code for one amino acid.
That code is translated into proteins ( each gene codes for one protein )
Each protein results in one trait (or is responsible for one part of one trait)
Proteins result in the physical characteristics
(traits) or enzymes (controls chemical reactions) of an organism

Transcription: Making mRNA from DNA
1.
One gene in the DNA molecule is opened
Quick-time movie

2.
RNA polymerase bonds free-floating
RNA nucleotides to open bases on the exposed strand of
DNA
Animation

Transcription Animation – Click on left hand box in top row
3.
The newly made mRNA leaves the nucleus and travels to the ribosomes in the cytoplasm

Translation: Making an amino acid chain from RNA -Animation
1.
The ribosomal subunits surround the mRNA

2.
tRNA binds to a specific amino acid and brings it to a specific codon on the mRNA. One end of the tRNA has 3 bases called an anticodon that is complementary to the codon on the mRNA.

3.
Amino acids are joined together by peptide bonds
4.
tRNA is released and reused

Video clip – 3:20
Animation

The Genetic Code
 This chart is used to determine which amino acid will be added based on the mRNA codon.

To find the amino acid that goes to a specific codon
(three-base mRNA sequence), find the first base on the left, the second base across the top, and then – within that box – the third base on the right.

Genetic
Code, cont’d
 Sometimes the chart for translating mRNA codons into amino acids is displayed in a circular format

 There are 64 possible codons
 There are only 20 amino acids
 Each codon codes for one amino acid
 Ex: CCC codes for proline
 Some amino acids may be coded for by multiple codons
 Ex: CCU, CCC, CCA, and CCG all code for proline
Video clip – 3:26

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There is one “ start
” codon –
AUG
– which codes for the amino acid methionine
There are three
“ stop
” codons
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These codons do not code for any amino acid
When a stop codon is reached, it is a signal that the amino acid chain is complete .
 This releases the amino acid – or polypeptide
– chain
 Also, the mRNA detaches from the ribosome and the ribosomal subunits come apart

Video clip – 1:47
Mutations
 Mutation = any error / mistake in the DNA
 Two types: point and frameshift
 Point mutations : those that affect one nucleotide – they occur at a single point on the DNA

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Substitution : changing one nucleotide for another
May have no noticeable effect if the codon still codes for the same amino acid

 May have a significant effect if the substituted codon changes the amino acid – video clip
– 0:59
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For instance, sickle cell anemia results from a single substitution point mutation
The change of one nitrogen base leads to a different amino acid, causing the range of problems faced by a person with sickle cell anemia

 Deletion – removing one or more nucleotides
 If a set of three nucleotides is removed, then one amino acid will be missing from the protein made by that gene The deletion shown above results in cystic fibrosis.

 If one or two nucleotides is deleted (or anything that is not a multiple of three), the deletion can change the amino acids for everything past the deletion

 Insertion
– adding one or more nucleotides
 If a set of three nucleotides is added, then the protein will have an additional amino acid

 If one or two nucleotides is added (or anything that is not a multiple of three), the insertion can change the amino acids for everything past the insertion

 Frameshift mutations : any changes that add or subtract nucleotides causing a change in the sequence of every codon after the mutation.
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This affects the “ reading frame
” –
AAT/CGG/ACC – if “T” is inserted after the first
“C”, it will now read AAT/CTG/GAC/C… changing all of the amino acids after the insertion

Frameshift Mutations
 Results when one or two nucleotides / bases are inserted or deleted
(or any number of bases that is not a multiple of 3)

Video clip
– 1:06
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 bacteria / viruses environmental / chemical
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UV radiation
Various pollutants genetic
 Some types of cancer (which is caused by a mutation in the DNA of the cells that become cancerous) can have a genetic predisposition
– making it more likely someone will get that type of cancer