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Unit 6: DNA Transcription and Translation

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nicholas waters

on 2 March 2018

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Transcript of Unit 6: DNA Transcription and Translation

Like DNA,
RNA is also a nucleic acid (polymer)
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
The protein that is produced by a "gene" is determined by the sequence of DNA nucleotide bases on the "coding" strand, but the mRNA that is built during transcription is derived from the "template" strand.
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.
What do proteins do again?
catalyze chemical reactions (amylase, pepsin, rubisco, hexokinase, etc.)
transport (hemoglobin)
structural support (keratin, collagen)
muscle movement (myosin, actin)
cellular communication (kinase)
1. A ribose sugar
2. A phosphate group
3. A nitrogenous base:
(Adenine, URACIL, Cytosine, or Guanine)
2. RNA polymerase "reads" the "template" strand of DNA in the 3' to 5' direction and adds RNA nucleotides to the growing RNA molecule in the 5' to 3' direction according to DNA/RNA base pairing rules.
Translation Intro
Unlike DNA, RNA is:
single stranded
uses ribose sugar instead of deoxyribose
uses the base Uracil instead of Thymine

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.
1. To create a protein that is made of 100 amino acids, how many codons would you need?

2. 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 ....

.... and there are 20 amino acids in nature

Living things decode the mRNA message (the genetic code) three nucleotides at a time.

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

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

If each "letter" (RNA nucleotide base) coded for a single amino acid, how many different amino acids could be coded for?

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.)

1. 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.
2. 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 - 3 bases at a time - in the 5' to 3' direction

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
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?
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.”

Translation is an appropriate name for the process because it converts the "genetic code" (nucleotides) into the "language" of proteins (amino acids)
A protein is a big molecule (polymer) that is formed when many amino acids (monomers) are bonded together
Primary structure = The sequence of amino acids in the protein (determined by the sequence of mRNA nucleotides)

The information in DNA cannot leave the nucleus...yet this information is needed by ribosomes - which are located outside of the nucleus - in order to make proteins.
Transcription happens in the nucleus
Transcription = DNA RNA
RNA processing
Typical Gene Structure
Introns & Exons
Intron = section of mRNA that does not get translated
Exon = section of mRNA that gets translated ("expressed") as a sequence of amino acids in a protein
Alternative "splicing" (why introns & exons?)
antibody 1
antibody 2
antibody 3
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
amino acid
Another term for protein is "polypeptide"
"Poly" = many
"Peptide" = bond that holds amino acids together
General Amino Acid Structure
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
Transfer RNA
Messenger RNA

CODONs: making translation possible
Methionene (MET)
The anti-codon of the tRNA forms complementary base-pairs with a specific three-base codon on the mRNA.
Transcription and Translation
Unit 6: Molecular Genetics:
...describes the flow of information in living things
(DNA to RNA)
(RNA to Protein)
Stated Clearly: "What Exactly is a Gene?"
(Or rather, what problem does it solve?)
...Transcription results in an RNA molecule that can leave the nucleus and deliver the information to the ribosomes
Life Science - protein synthesis (translation)
amoeba sisters: protein synthesis (updated)
The Central Dogma: DNA Learning Center
Thus, the base-pairing rules for making RNA from DNA are:

A U *(Adenine bonds with Uracil)
C G (Cytosine bonds with Guanine)
DNA Learning Center: Transcription
Transcription in Context
Transcription takes place in the nucleus
made of RNA nucleotide monomers
***The Result is an mRNA molecule that looks exactly like the "coding strand" of DNA except that the RNA molecule has uracil (U) bases instead of thymine (T)
To make sense of transcription, we need to understand that:

DNA has a
1. "coding" strand and a
2. "template strand"
Steps of Transcription
1. Transcription factor proteins and RNA polymerase bind to the promoter region of a gene
3. RNA processing:
3a. splicing

3b. 5' methyl "cap" added

(introns removed and exons pasted together by a "spliceosome"
- 3c. 3' poly A "tail" added
(cap is a modified guanine base, which protects the mRNA and helps the ribosome attach
(tail is a bunch of adenosine bases, which protects the 3' end of the mRNA)
4. Alternative Splicing of the exons
***only the exons contribute to the final protein
5' "Cap"
Poly A Tail
Alternative splicing allows multiple proteins to be produced from a single gene
Each 3-nucleotide section in mRNA is known as a codon
Practice ...
DNA Learning Center: Translation
Because there are 20 different amino acids, how many different tRNA's must exist?
Remember this: there are 64 possible codons - three of which are "stop" codons that do not specify an amino acid.

With this in mind, how many tRNAs should there be?
In reality, there are somewhere between 30 and 41 tRNAs

(some tRNAs are capable of matching multiple codons)
Protein Database
Human Protein Atlas (proteome project)
Molecular Machines needed to make life possible
DNA Polymerase
RNA Polymerase
aminoacyl tRNA synthetases
Peptide bond = type of bond that joins amino acids together, forming a polypeptide
Protein Structure
* Ultimately, the sequence of amino acids determines the protein's 3-dimensional shape - and therefore - function
Secondary structure = hydrogen bonds form among different amino acids to form "alpha helices" or "beta sheets"

Tertiary structure = "alpha helices" and "beta sheets" interact with each other and fold

Quaternary structure = multiple amino acid chains interacting together

Insulin Protein
Hemoglobin Protein
transports oxygen
regulates blood sugar
Codon Chart
Full transcript