http://www.pbs.org/wgbh/nova/education/body/rnai-discovered.html

Recipe/chef dna to protein

Unit 5A

Gene Expression

Review

• What is a gene?– A segment of DNA that codes for a specific

protein

• What do you think it means to say “express a gene”?– Gene expression- The process by which

information in a gene is used to create the protein that it codes for.

How does the cell “decode” the sequence of DNA bases of a gene?

• In general:

The master copy of instructions

The temporary copy of the DNA

The product of the DNA’s instructions

DNA RNA Protein Trait

Characteristic of a cell/organism

RNA

• A nucleic acid• Consists of one long chain of nucleotides• There are multiple types of RNA, each

with a different function• One type of RNA can be used as a

temporary copy of DNA in order to make protein

• The DNA is precious and stays safely in the nucleus

DNA vs. RNA

• RNA is mostly single stranded

• RNA is made of nucleotides that have a phosphate, ribose sugar and a base

• Bases in RNA are A, U, C, G– U = uracil– What’s missing?

Compare and Contrast DNA and RNA Structure

DNA RNA

RNA is used in Protein Synthesis

• Proteins are made of __________.

• RNA molecules direct the assembly of amino acids into _____________.

• Which then fold into proteins.

Amino acids

Polypeptides

Types of RNA

• Messenger RNA (mRNA)- a copy of the instructions for making protein from the DNA

Types of RNA

• Ribosomal RNA (rRNA)- RNA molecules that bind with proteins to make the ribosomes (where proteins are made)

Types of RNA

• Transfer RNA (tRNA)- transfers the correct amino acid to the ribosome to make proteins based on the mRNA sequence

The “Central Dogma” of Molecular Biology

• First DNA is transcribed into mRNA• Then, the mRNA is sent out of the nucleus

to a ribosome• At the ribosome, tRNAs bring together

amino acids to translate the message in the mRNA into protein.

The master copy of instructions

The temporary copy of the DNA

The product of the DNA’s instructions

DNA RNA Protein

DNA to RNA to Protein

2 processes are required for gene expression

1.Transcription- the process that uses base pairing to make mRNA based on the sequence of DNA – Occurs in the nucleus

2. Translation decodes the mRNA message into protein– Occurs at the ribosome (in the cytoplasm)

Draw in diagram from DNA -> Secreted Protein

Making RNA

1. Transcription- DNA serves as the template for making complementary RNA molecules

• RNA is made in the nucleus, then moves to the cytoplasm for protein synthesis

• RNA polymerase separates the DNA strands and assembles RNA by matching up base pairs

Making RNA What pairs with adenine in the DNA when making the new RNA molecule?

DNA to mRNA practice (transcription)

• Transcribe the following DNA sequence (gene)• TACAATGGCGCTAGT

• The DNA is ultimately read three letters at a time or we say in “triplets”

DNA Triplet code

Making a Polypeptide

2. Translation decodes the mRNA message into protein– Occurs at the ribosomes– Remember: the ribosome is

made of two subunits of rRNA and proteins

The Genetic Code:

• The language spelled out by the sequence of bases on the mRNA (A, U, C, G)

• Read 3 letters at a time

• Each 3 letter “word” is called a codon

• Each codon corresponds to 1 amino acid

Amino acids

DNA Triplet code

mRNA codons U - U - A - C - A - G - C - C - A

Transcription

Translation

How to Read Codons• There are 64 possible

codons• The genetic code table

1. Start in the middle of the circle with the first letter of the mRNA codon

2. Move to the second ring to find the second letter of the codon

3. Find the third letter of the codon in the smallest set of letters

4. Read the amino acid in that sector

Amino Acids

• There are 20 amino acids commonly found in proteins

• A string of amino acids is a polypeptide

• A completed and folded polypeptide is a protein

3 letter abbreviation

1 letter abbreviation

Start and Stop Codons

• Genetic punctuation marks

• AUG is the start codon– codes for the amino acid methionine

• The mRNA is read starting at AUG

• After the mRNA is fully read, a stop codon ends the production of the protein

• Three stop codons: UAA, UAG, UGA– No amino acids are coded

mRNA to Protein Practice

• AUGUUACCGCGAUCA

How does the cell read the codons? 1. The ribosome attaches to a mRNA molecule that has

entered the cytoplasm2. As each codon on the mRNA passes through the

ribosome, tRNAs bring the proper amino acids into the ribosome

3. The ribosome attaches these amino acids to each other with peptide bonds creating a polypeptide

4. This occurs until the stop codon enters the ribosome-at that point, the polypeptide is complete and is released from the ribosome

5. After the polypeptide is released, it finishes folding up properly into a fully functioning protein

VideoVideo

How does tRNA know which amino acids to bring?

• Each tRNA carries only 1 kind of amino acid

• Each tRNA has 3 unpaired bases that match up with the codons on the mRNA– These are called

anticodons

anticodon

The Genetic Code is Nearly Universal!!

• Almost all organisms have the same three letter codes in DNA leading to the same amino acids brought in to make the proteins.

• From bacteria to humans, the triplet code is the same.

DNA (gene):

TGA CGA TTT CTC ACT ACA CGC GCC CTT

mRNA

PolypeptideTranslation↓

Transcription↓

An different way to model protein synthesis

Stanford’s take on Protein Synthesis

Mutations

Mutations

• Mistakes in copied DNA that cause variations in genetic material

• These mistakes are passed down to offspring if they occur in sex cells

Overactive feather blocking gene creates a bald neck

Mistake in the “bithorax” gene causes 2 sets of wings

2 Basic Categories

1.Gene mutations- changes in a single gene

2.Chromosomal mutations- changes in a whole chromosome

1. Gene Mutations

• Are point mutations- changes at a single point in the sequence

• Occur during replication

• Are passed down to daughter cells

• 3 types:– Substitutions– Insertions– Deletions

Substitutions• One base is changed to a

different base• Affects a single amino

acid• Sometimes have no effect

at all– How could this happen?– If the substitution creates a

codon that leads to the same amino acid as before

Insertions

• One extra base is inserted into the DNA sequence

• Every codon after the insertion will change!

• Considered a “frameshift” mutation because they shift the reading frame of the genetic message

• The amino acids chosen will change

• Will the resulting protein work?– Very unlikely!

Deletions

• One base is deleted from the DNA sequence

• Every codon after will change

• Also a “frameshift” mutation

• Changes the amino acid sequence and resulting protein

AnalogyRead the following sentence (remember, it’s read in triplets): The fat cat ate the rat.Insertion:• Add a b in front of the f in fat. Shift everything to the right.• _____ _____ _____ _____ _____ _____Deletion:• Delete the f in fat. Shift everything to the left.• _____ _____ _____ _____ _____ _____Substitution:• Substitute the letter b for the f in the word fat and read the sentence

again.• _____ _____ _____ _____ _____ _____• Which change does the least damage to the sentence structure?

____________________• What does this tell you about the mutation types?

2. Chromosomal Mutations

• Changes in the number of chromosomes or the structure of chromosomes

• Can change the location of genes on a chromosomes

• Can change the number of copies of a gene

4 Types of Chromosomal Mutations

• Deletion- loss of part of chromosome

• Duplication- extra copy made of part of chromosome

• Inversion- reverses direction of part of chromosome

• Translocation- part of one attaches to another chromosome

Causes of Mutations

• Mistakes in replication

• Mutagens- chemical or physical environmental agents– Pesticides– Tobacco smoke– X-rays– UV rays

Mutations are Harmful and Helpful

• Harmful- if the change results in a nonfunctional protein

• Ex: a point mutation in the hemoglobin gene results in sickle cell anemia

Sickle Cell Anemia

• A nucleotide substitution causes the hemoglobin protein to change shape and no longer carry oxygen efficiently

• This causes the rigid, sickle shape in the red blood cells

• These cells get caught in capillaries and cause pain and other health problems

Sickle Cell Anemia

Mutations are Harmful and Helpful

• Helpful- produces a better functioning protein for a certain environment

• Mutations can lead to evolution

• Example: There is a human mutation that makes people have increased resistance to HIV

Cellular Specialization

• When replication takes place, is all of the DNA in a cell copied?

• Is the DNA in different cells of your body exactly the same?

• If the DNA in every cell is the same, how do cells become different from each other?

• The light switch analogy: – Cells “turn on,” or transcribe and translate only

those genes that code for the proteins they need. Other genes are “turned off” because the proteins they code for are not needed by the cell.

Cellular Specialization ExampleThe following is a short sequence of DNA. Four different imaginary genes

are shown. The protein for each gene is listed under the code.

WBC Skin Cell Muscle Cell Nerve Cell

ON__________ ON___________ ON___________ ON_____________

OFF_________ OFF___________ OFF___________ OFF_________

Given the following cells, list the genes above that would be “switched on” or expressed (active) in the cell and list those that would be “switched off” or not expressed (inactive)?

Coding vs. Non-coding DNA

• Coding DNA: areas of DNA that code for proteins, also known as ___________. – ~2% of the genome

• Non-coding DNA: stretches of DNA that do not code for protein– What does the other ~98% of the DNA do then?– Some areas regulate/control the genes, other areas

are genes that no longer function (i.e. the genes for a tail), etc.

– The function of much of the non-coding DNA is still unknown

genes

Is Complexity Dictated by the Non-Coding DNA?