Protein Synthesis
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A gene is a segment of DNA that is located on a chromosome and codes for a hereditary characteristic.
The gene directs the making of the protein melanin in hair follicle cells.
Transcription-The synthesis of RNA from DNA
Translation – RNA directs the assembly of proteins
Protein synthesis- Forming proteins based on information in DNA and carried out by RNA
RNA Structure
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RNA contains the sugar ribose instead of deoxyribose
RNA contains the nitrogenous base uracil in place of thymine
RNA is usually single stranded not double stranded
RNA is much shorter than DNA, usually the length of one gene
Adenine pairs with uracil and cytosine pairs with guanine
Types of RNA
Cells have three major types of RNA. Each plays a different role in protein synthesis.
1. messenger RNA (mRNA)- a single stranded molecule that carries the instructions from a gene to make
a protein. It carries the genetic message from DNA to the ribosomes.
2. ribosomal RNA (rRNA)- It is part of the structure of the ribosome and help in building the structure of
proteins.
3. transfer RNA (tRNA)- They transfer amino acids from the cytosol to the ribosomes.
Transcription
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The process by which the genetic instructions in a specific gene are transcribed into an RNA molecule.
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It takes place in the nucleus of eukaryotic cells and in the cytosol of prokaryotic cells
Steps of Transcription
1. RNA polymerase binds to a promoter on DNA. A promoter is specific nucleotide sequence of
DNA.
2. When RNA polymerase binds to DNA it unwinds and unzips the area that needs to be
copied.
3. RNA polymerase adds free RNA nucleotides that are complementary to the nucleotides on
one side of the DNA strand. The resulting chain is an RNA molecule.
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4. If the DNA strand is ATCGAC the corresponding RNA strand would be UAGCUG.
5. As RNA polymerase passes a DNA region during transcription the region rewinds.
6. When RNA polymerase gets to the end of a gene it reaches a termination signal, a specific
sequence of nucleotides that marks the end of a gene.
7. Upon reaching a stop signal, RNA polymerase releases the DNA and newly formed RNA
molecules.
8. Transcription can make mRNA, rRNA, and tRNA.
The Genetic Code
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Three adjacent nucleotides in mRNA specify an amino acid.
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Each group of three nucleotide sequence in mRNA is called a codon.
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There are 64 mRNA codons. The genetic code is nearly universal to all living things and supports the
idea that all organisms share an ancient common ancestor.
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Some amino acids are encoded by 2,3,or more different codons.
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No codon encodes more than one amino acid.
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AUG is the amino acid methionine and also is the start codon and tells a ribosome to start translation.
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UAA, UAG, and UGA are stop codons and tell the ribosome to stop translation. They do not code for an
amino acid.
Translation- the decoding of the genetic instructions to make a protein
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All three types of RNA are involved in making a protein.
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Every protein is made of one or more polypeptides.
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Polypeptides are chains of amino acids. There are 20 different amino acids found in the proteins of
living things.
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Each protein may consist of hundreds or thousands of the 20 different amino acids arranged in a
specific to each protein.
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The amino acid sequence will determine the proteins three dimensional shape.
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The shape of a protein determines its function.
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Steps of Translation
1. Initiation
a. mRNA enters the ribosome.
b. rRNA reads the mRNA strand and assists in the assembly of proteins
c. tRNA has a 3 nucleotide anticodon on one end and its corresponding amino acid attached to
its other end. It gets the amino acid from the cytosol.
d. tRNA carrying the amino acid methionine at one end and the anticodon UAC on the other
end pairs with the start codon AUG on the mRNA.
e. The first amino acid in all polypeptides is methionine but it can be removed later.
2. Elongation
a. A tRNA carrying the appropriate amino acid pairs its anticodon with the second mRNA
codon.
b. The ribosome then detaches methionine from the first tRNA and a peptide bond forms
between methionine and the second amino acid.
c. The first tRNA then exits the ribosome.
d. The ribosome then moves a distance of one codon along mRNA.
e. Steps a through d continues to add amino acids lengthening the polypeptide.
3. Termination
a. The process ends when a stop codon is reached.
b. A stop codon is one for which there is no tRNA that has a complementary anticodon.
4. Disassembly
a. The newly made polypeptide is released
b. The last tRNA leaves the ribosome
c. The ribosome moves away from the mRNA
*Several ribosomes may translate the same mRNA transcript at the same time. As a ribosome moves along to
the next mRNA codon another ribosome can attach to the same mRNA. This allows a ribosome to make many
proteins in a short period of time.
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The Human Genome
Genome- entire gene sequence, complete genetic content
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There are 3.2 billion base pairs in 23 chromosomes
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There are approximately 30,000 genes
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Each gene codes for a protein
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We know what some of the genes control. Having this knowledge may help us to diagnose,
treat, and prevent genetic disorders, cancer, and infectious diseases.
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