Protein Synthesis: The Central Dogma of Biology

Chapter 8 in your textbook

RNA: Ribonucleic acid Used to make proteins

DNA RNA

Thymine Uracil

Double – stranded Single – stranded

1 type 3 types

Deoxyribose sugar Ribose sugar

Three Types of RNA1. 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)

Video clip – 2:52

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

The Central Dogma of Biology

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

Transcription2. RNA polymerase

bonds free-floating RNA nucleotides to open bases on the exposed strand of DNA

Animation

Transcription

3.The newly made mRNA leaves the nucleus and travels to the ribosomes in the cytoplasm

Animation – Click on left hand box in top row

Translation: Making an amino acid chain from RNA -- Animation

1. The ribosomal subunits surround the mRNA

Translation, cont’d2. 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.

Translation, cont’d

3. Amino acids are joined together by peptide bonds

4. tRNA is released and reused

Protein Synthesis

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.

The Genetic Code, cont’dTo 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

The Genetic Code, cont’d 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

prolineVideo clip – 3:26

The Genetic Code, cont’d There is one “start” codon – AUG – which

codes for the amino acid methionine There are three “stop” codons

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

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

Video clip – 1:47

Point Mutations Substitution:

changing one nucleotide for another

May have no noticeable effect if the codon still codes for the same amino acid

Point Mutation - Substitution May have a significant effect if the substituted

codon changes the amino acid – video clip – 0:59

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

Deletions 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.

Deletions, cont’d 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

Insertions Insertion – adding one or more nucleotides

If a set of three nucleotides is added, then the protein will have an additional amino acid

Insertions, cont’d 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 Frameshift mutations: any changes that

add or subtract nucleotides causing a change in the sequence of every codon after the mutation. 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)

Causes of Mutations

bacteria / viruses environmental / chemical

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

Video clip – 1:06