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Unit 6: DNA Transcription and Translation

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nicholas waters

on 6 February 2014

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Transcript of Unit 6: DNA Transcription and Translation

Like DNA, RNA is also a nucleic acid
An RNA nucleotide is composed of three parts:
There are three main types of RNA that can be produced during transcription:

1. Messenger RNA (mRNA): codes for proteins
*(serves as a disposable blueprint of the
genetic code)

2. Ribosomal RNA (rRNA): is a component of ribosomes

3. Transfer RNA (tRNA): adds amino acids to a growing protein
A gene is a section of DNA that codes for a specific protein
The protein that is produced by a "gene" is determined by the sequence of DNA nucleotide bases in the gene
To go from DNA to a protein, the genetic code must be first "transcribed" into RNA (RiboNucleic Acid) and then "translated" into a protein.

DNA RNA = Transcription

RNA (Ribonucleic acid) Protein = Translation
Transcription is the process by which the genetic instructions in a gene are transcribed or "rewritten" into an RNA molecule
Translation is the process of decoding the genetic information contained in a mRNA molecule into a protein.
How does a gene relate to DNA? To a chromosome?
http://learn.genetics.utah.edu/content/begin/dna/firefly/
What do proteins do again?
catalyze chemical reactions (amylase, pepsin, rubisco, hexokinase, etc.)
transport (hemoglobin)
structural support (keratin, collagen)
antibodies
muscle movement (myosin, actin)
hormones
cellular communication (kinase)
1. A ribose sugar
2. A phosphate group
3. A nitrogenous base:
(Adenine, URACIL, Cytosine, or Guanine)
What, then, are the main differences between DNA and RNA?
During transcription, RNA polymerase (an enzyme) binds to the "template" strand of DNA and adds RNA nucleotides to the growing RNA molecule according to DNA/RNA base pairing rules.
Based on this diagram of a cell, where does transcription take place within the cell?
During transcription, the single-stranded molecule of RNA that forms is ALMOST identical to the coding strand of DNA
DNA:
Transcription and Translation

DNA
RNA
= ?
Transcription
Translation Intro
Unlike DNA, RNA is single stranded, is made using ribose instead of deoxyribose, and uses the base Uracil instead of Thymine
Thus, the base-pairing rules for making RNA from DNA are:

A U *(Adenine bonds with Uracil)
C G (Cytosine bonds with Guanine)
What is the only difference between the coding strand of DNA and the RNA molecule that is transcribed?

1. Messenger RNA (mRNA): codes for proteins
*(serves as a blueprint of the genetic code)

2. Ribosomal RNA (rRNA): is a component of ribosomes

3. Transfer RNA (tRNA): adds amino acids to a growing protein
In order to make a protein from the information contained in DNA, three types of RNA are needed:
There are 20 different amino acids that can be combined in different ways to make the 1,000's o proteins that keep organisms alive.
(To create a protein that is made of 100 amino acids, how many codons would you need?


How many nucleotides would be needed for that many codons?)
When making a protein, there is a problem that the cell encounters...

There are only four possible letters in the genetic code of an mRNA molecule: A,U,C,G

However, there are 20 amino acids

If each "letter" (RNA nucleotide base) coded for a single amino acid, how many different amino acids could be coded for?
The solution to the problem is that living things decode the mRNA message (the genetic code) three nucleotides at a time.

Each 3-letter "word" in mRNA is known as a codon

Because there are four different "letters" (nucleotide bases) in the mRNA code, and the code s read three "letters" at a time, there are 64 possible codons (4x4x4) --
AUG, AGU, CCC, etc.

Each of these 64 codons "codes" for an amino acid
How do we get from RNA to protein?
A codon is a group of three nucleotides on a messenger RNA molecule that specify a single amino acid.


If each letter of the mRNA coded for an amino acid, only four amino acids could be coded for.

However, the cell needs to code for 20 different amino acids.

Why is transcription important and necessary?
How is an RNA nucleotide different from a DNA nucleotide?
Why is translation important?

Each tRNA carries only one type of amino acid depending on the anti-codon.

(For example some tRNA's carry only methionine, others only carry valine, etc.)

because there are 20 different amino acids, how many different tRNA's must exist? (how many codons are there?)
Translation begins when an mRNA molecule in the cytoplasm attaches to a ribosome.
AUG is known as the "start codon" because this is where the ribosome begins reading the mRNA molecule.
AUG (the start codon) codes for the amino acid methionine.
As each codon of the mRNA molecule is "read" by the ribosome, the proper amino acid is brought into the ribosome by a transfer RNA ( "tRNA" ).
A tRNA has two main components:

2. an amino acid that compliments the mRNA codon

1. an anti-codon that forms complementary base pairs with the mRNA codon



The ribosome has three places for tRNA:

1. A place for tRNA to enter (
A site
)

2. A place to form a peptide bond between the amino acids carried on the tRNA molecules (
P site
)


3. A place for tRNA to exit (
E site
)
In summary, translation is a three step process:

1. The mRNA enters the ribosome and the ribosome reads the mRNA code

2. a tRNA delivers the appropriate amino acid specified by the mRNA code... (
An anticodon on a tRNA pairs with the codon on the mRNA
).

3. A peptide bond is formed between the amino acids that are carried by the tRNA's
Finally...Translation
Each cell in an organism needs DNA to function. If an organism starts off as one cell, how do the millions of cells in an adult organism get that same DNA?
Problem 1
Problem 2
DNA contains the information needed to make proteins. However, DNA is trapped in the nucleus and proteins are made in ribosomes that are in the cytoplasm.

How can the information in DNA get out to the ribosome?
Problem 3
The cell can't really "use" RNA to perform any of the functions that will keep it alive. How does the information contained in RNA get turned into something useful for the cell?
The Result is an RNA molecule that looks exactly like the coding strand of DNA except that the RNA molecule has uracil (U) bases instead of thymine (T)
coding strand
template strand
RNA molecule
Because the genetic code is so important for the survival of each cell, the DNA never leaves the protection of the nucleus. But, proteins are made in the cytoplasm…
Translation takes place within ribosomes. These ribosomes are either free-floating in the cytoplasm or are attached to the endoplasmic reticulum
Cells speak the language of “protein.” Thus, translation is like de-coding a secret message because the information in a mRNA molecule cannot be used by the cell until it is de-coded (translated) into a protein
A protein is a big molecule (polymer) that is formed when many amino acids (monomers) are bonded together
The sequence of amino acids will determine the 3-dimensional shape of the protein. The shape of the protein determines its function. Therefore, the correct sequence of amino acids in a protein is very important for a protein to function correctly
The information in DNA cannot leave the nucleus. Transcription results in an RNA molecule that can leave the nucleus and deliver information necessary to make proteins to the ribosomes
Transcription happens in the nucleus
Transcription = DNA RNA
tRNA
mRNA
rRNA
A U
C G
mRNA processing
Typical Gene Structure
Introns & Exons
Intron = section of mRNA that does not get translated into an amino acid sequence
Exon = section of mRNA that gets translated ("expressed") into an amino acid sequence
Alternative "splicing" (why introns & exons?)
antibody 1
antibody 2
antibody 3
Why is transcription an accurate term for this process?
Cells cannot "use" the language of nucleotides (A,T,C,G,U) to do work. Translation converts the language of nucleotides into the language of amino acids...
Amino acids bond together to form proteins that can perform the jobs needed to keep cells and organisms alive
Ribosome
amino acid
protein
Another term for protein is "polypeptide"
"Poly" = many
"Peptide" = bond that holds amino acids together
General Amino Acid Structure
mRNA
Proteins
Ribosomes read the code on the mRNA three nucleotide bases at a time and turn that information into a sequence of amino acids
Ribosomal RNA
rRNA
Transfer RNA
tRNA
Messenger RNA
mRNA
Ribosomes and RNA
A problem with Translation
AUG
Methionene (MET)
The anti-codon of the tRNA forms complementary base-pairs with a specific three-base codon on the mRNA.
Full transcript