Gene Gene ExpressionExpression

Transcription and Translation

WHAT IS A GENE?WHAT IS A GENE?• Gene: • 1) A unit of hereditary

information which can be passed on to future generations.

• 2) A segment of DNA on a chromosome which holds the code or “recipe” for a protein.

• Genes hold the code for synthesis of

• proteins : A functional group of many amino acids.

• polypeptides : A smaller, non-functional group of amino acids

• traits or parts of a trait: any physical, physiological or behavioral trait.

Types of Types of TraitsTraitsPHYSICALBEHAVIORALPHYSIOLOGICAL

Eye Color

Hair Color

Review of the amino acid structure of proteins: Circle R groups and show aa’s. Identify as polypeptide.

How many different TYPES of amino acids could be used to form a single protein or polypeptide? _20_

Proteins come in many forms. Due to the variety of different proteins, they serve many different functions that contribute to the growth, maintenance and replacement of cells.

Type of Protein Name of Protein Exact Function

Structural Collagen In structure of skin tissues

Keratin In structure of hair, bones, skin and nails

Contractile Myosin, actin In muscle contraction

Transport Hemoglobin In carrying oxygen in blood

Hormones Insulin, glucagon In control of blood sugars

Enzymes Pepsin, amylase In digestion of organic compounds

DNA polymerase Replication or duplication of DNA

Protective Gamma globulin In antibody formation

Fibrinogen In blood clotting

Toxins Venom Used for protection or to capture prey

PROTEINSPROTEINSIn other words, we need proteins to

carry out EVERY SINGLE trait. ◦ Movement◦ Cell respiration◦ Photosynthesis◦ Hydrolysis◦ Dehydration Synthesis◦ Cardio/Respiratory/Digestive functions◦ Eye color◦ Hair color◦ Skin color◦ Physical Features◦ EVERYTHING………..

Eye Color

Hair Color

The DNA CodeThe DNA Code• A gene holds the code for a functional

protein or polypeptide. The code is found in the sequence of nucleotides on one strand of the DNA at a specific location on a chromosome.

• In 1961, Francis Crick and coworkers proposed a triplet code for the formation of amino acids from DNA.

• A set of 3 nucleotides codes for 1 amino acid

• For example: the amino acid methionine is coded for by the DNA nucleotide triplet TAC.

• The sequence of triplet codes then codes for a protein of polypeptide.

Section of a gene which codes for a polypeptide: Notice that three bases in the DNA code for a single amino

acid. Identify PO4, sugars, N bases, and aa’s.

Gene is read from one of the two DNA strands.

• If a protein is made of 100 amino acids, how many bases in the DNA are needed to code for the protein?

• more than 300 nitrogen bases• (303 with stop code and >303 with

introns)

• Universality of the genetic code: • ALL organisms use the same triplet code

to represent the same amino acids.• ex. whales, spiders, and bacteria-but

they don’t necessarily make the same proteins

Protein Synthesis Protein Synthesis or theor the building of building of proteins.proteins.• Proteins are synthesized by ribosomes in the

cytoplasm and on the rough ER. • Where is DNA located? nucleus of a

eukaryotic cell• Additional molecules will be needed to carry

the DNA code to the cytoplasm and form proteins.

• This molecule is called RNA (Ribonucleic acid)

• RNA nucleotides are composed of : • 1) ribose sugar• 2) PO4• 3) and one of four nitrogen bases: adenine,

guanine, cytosine and URACIL (takes the place of thymine in RNA) .

Differences between DNA and RNA:Characteristic DNA RNA

Location in the cell found only in the nucleus

Found in nucleus and cytoplasm

Type of sugar in nucleotide

deoxyribose ribose

Number of strands 2 1

Nitrogen bases A,G,C, thymine--no uracil

A,G,C, uracil—no thymine

Can be copied or replicated

yes no

Different forms only one form—DNA mRNA, tRNA and rRNA

• 3 types of RNA necessary for synthesis of proteins:

• m RNA-messenger RNA: carries an RNA “copy” of the DNA gene code from the nucleus to the cytoplasm.

• t RNA-transfer RNA: transports amino acids from the cytoplasm to the ribosome so they can be joined to make a protein.

• r RNA-ribosomal RNA __molecules of rRNA make up the ribosomes 2 units plus associated proteins.

GENE EXPRESSIONGENE EXPRESSION GENE EXPRESSION - How the genes are

expressed is a function of the protein produced. DNA (gene) m RNA Protein Trait

(expression)

Steps in the Synthesis of Steps in the Synthesis of a Protein:a Protein:1) Transcription: mRNA strand is built from the code on the DNA strand (gene)

2) Translation: mRNA strand is used at the ribosome to help join amino acids to form a protein

Draw in diagram from Draw in diagram from DNA DNA Secreted Protein Secreted Protein

I. Transcription:I. Transcription:Requirements for Transcription:

a gene segment on the DNA

many free floating RNA nucleotides and

the enzyme RNA polymerase

Steps in transcription:Steps in transcription:• DNA double helix unwinds at the area of

the DNA where the gene is located.

• Weak hydrogen bonds break between the 2 DNA strands.

• Free floating RNA nucleotides bond with complimentary DNA nucleotides to form a strand of mRNA. These stages are catalyzed by the enzyme RNA polymerase.

• When the mRNA is completely formed, it breaks away from the DNA. The DNA strands reconnect and the mRNA is free to travel to the cytoplasm.

Remember Remember that in that in

transcription, transcription, the the

complimentacomplimentary base pair ry base pair

for adenine is for adenine is uracil NOT uracil NOT thymine!thymine!

• Each triplet _code (3 nucleotides) on a DNA molecule is transcribed into a triplet __codon_ on the mRNA molecule.

• If the DNA codes for a polypeptide is• T—A—C—C—C—G—T—A—G—C—T—T—A—C—T • What would the codons on the complimentary

strand of mRNA codons look like?AUG – GGC – AUC – GAA - UGA

• Example:• DNA codes:• T A C C A T C C C A A A A

C T• mRNA codons:

AUG – GUA – GGG – UUU - UGA

• What does each letter in the code or codon stand for represent?

• a nucleotide

• What do the particular codes and codons represent?

• amino acids

• Because each base triplet on the mRNA stands for an amino acid, each mRNA molecule must contain

• 3X as many nucleotides as amino acids

• 1) An mRNA strand with 66 nucleotides codes for a polypeptide 22 amino acids in length.

• 2) A protein 300 amino acid units in length was synthesized from an mRNA strand 900 nucleotides in length.

• A DNA strand 700 nucleotides in length will be transcribed into an mRNA strand 700 nucleotides

• There are 4 different bases in DNA (A, C, G, C) which can combine in different triplets to form 64 possible triplet codes (43). When these codes are transcribed into mRNA, there are 64 triplet codons which can be formed. Only 20 amino acids are used so some codons represent more than one amino acid.

There are 4 different bases in DNA (A, C, G, C) which can combine in different triplets to form 64 possible triplet codes (43). When these codes are transcribed into mRNA, there are 64 triplet codons which can be formed. Only 20 amino acids are used so some codons represent more than one amino acid.

mRNA Codon ChartmRNA Codon Chart

• How many different mRNA codons are there on the chart?

• 43 = 4 x 4 x 4 = 64 different codons (using A, G, C and U in triplets)

• How many different amino acids are there? Only 20

• With the exception of Methionine and Tryptophan, more than one nucleotide triplet codes for each amino acid.

• Give an example of an amino acid which has more than one codon.

• valine, alanine, isoleucine, threonine, and more

• What do you notice about the codons that code for the same amino acid?

• They are very similar and usually only the last nitrogen base is different.

• Some codons have special functions:• Start Codon: • AUG-codes for the amino acid methionine. • It signals the ribosome that it is the

beginning of the mRNA sequence.

• Stop Codons:UAA, UAG, UGA• Don’t code for an amino acid!!!! • Signal the ribosome that it is the end of

the mRNA sequence and triggers the release of the mRNA from the ribosome

• What is the DNA code for the mRNA start codon, methionine? TAC

• What are the DNA codes for the stop codons UAA, UAG and UGA? ATT, ATC, ACT

After transcription has taken place, the mRNA moves out of the nucleus to link with a ribosome in the cytoplasm. Translation will take place here.

II. Translation:II. Translation:• Process of using mRNA codons to build proteins

at the site of the ribosomes in the cytoplasm or on the rough endoplasmic reticulum.

• Requirements for translation:• m RNA __carries the DNA code as “codons”• ribosomes __2 rRNA+ protein subunits--site

of protein synthesis• tRNA __carry the amino acids from the

cytoplasm to the ribosome to be joined to form the protein

• Abundant amino acids in the cytoplasm and all necessary enzymes.

Steps in Translation:Steps in Translation:

• mRNA joins with the __ribosome (in cytoplasm or near the Rough Endoplasmic Reticulum)

• tRNA (with an amino acid attached) _joins the mRNA codon to ANTI-codon

• (The anticodon on the tRNA is complimentary to the codon on the mRNA.)

• a second tRNA with attached amino acid _joins the mRNA

• a peptide bond forms between the first two amino acids

Steps in Translation:Steps in Translation:

• the first tRNA is released from the mRNA _amino acids stay bonded to each other

• the mRNA slides through the ribosome _to expose the next codon_

• the next tRNA with amino acid _joins the mRNA• Sequence is repeated Repeat steps 4-8 until a

stop codon is reached.• mRNA and protein is released from the ribosome.

The Final Products of The Final Products of Protein Synthesis:Protein Synthesis:• protein or polypeptide: __functional or non-

functional sequence of aa’s• Many times a protein must be modified after it is

translated. This is done in the _cytoplasm, ER, Golgi____

• Some proteins are coded for by a single gene • ex. Insulin (later broken into two segments)

• Sometimes proteins are composed of different gene• hemoglobin (2 genes/4 segments)

GENE MUTATIONSGENE MUTATIONS• GENE MUTATIONS: Any change in the normal sequence of nucleotides in a gene sequence.

• There are different variations.• can be harmful / helpful / have no effect

• can occur spontaneously during DNA replication

• (despite enzymes that help to check the DNA)

• Many of the mutations in DNA are caused by mutagens. Some of these mutagens are:

• (1) ultraviolet light (can cause skin cancer)

• (2) chemicals (like those found in cigarette smoke)

• (3) viruses (like HPV: (human papilloma virus)

• (4) X-rays, gamma rays (high energy-given off in atomic blast- formerly used safely in medical procedures)

• An altered gene may be passed on to every cell that develops from it BUT it will be carried on to the next generation only if the mutation takes place in a sperm or egg cell.

• * If a mutation occurs in a gene it is usually harmful. It can cause diseases such as cancer, and many hereditary diseases that are passed from parent to offspring.

• If you inherit a disease such as hemophilia, what are you actually inheriting?

• You are inheriting an altered gene sequence that produces a non-functioning protein or a protein that does not function normally.

• There are several ways that the base sequence can be changed, or mutated:

• (1) __ADDITION of an extra nucleotide within the sequence

• (2) __DELETION of a nucleotide within the sequence

• (3) __SUBSTITUTION of a different nucleotide for one normally present in the sequence

• Analogy: Read the following sentence (remember, it’s read in triplets):

The fat cat ate the rat.

• Addition:• Add a b in front of the f in fat. Shift everything to the

right.• The bfa tca tat eth era t

 • Deletion:

• Delete the f in fat. Shift everything to the left.• The atc ata tet her ra

 • Substitution:• Substitute the letter b for the f in the word fat and

read the sentence again.• The bat cat ate the rat.

 • Which change does the least damage to the sentence

structure? substitution

INSERTION MUTATION

DELETION MUTATION

SUBSTITUTION MUTATION

• An example of a disease caused by a gene mutation is the disease Sickle Cell Anemia.

• What is sickle-cell anemia?__a nucleotide substitution which results in hemoglobin molecules that become rigid, causing the red blood cell to assume a half-moon or ‘sickle’ shape. The cells do not carry oxygen as effectively and can have difficulty moving through blood vessels. Causes debilitating pain, strokes, eye damage and shortened life span.

•Because the gene sequence is mutated, the gene expression is altered.

Cellular SpecializationCellular Specialization• When replication takes place, is all of the DNA in

a cell copied? _yes________________•  • Is the DNA in different cells of your body exactly

the same? _yes__________________________________

•  • 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.____

The following is a short sequence of DNA. Four different imaginary genes are shown. The protein for each gene is listed under the code.

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)?

Gene 1: Skin Pigment Gene 2: muscle proteins Gene 3: antibody proteins (immune defense) Gene 4: Control enzyme for cellular respiration

WBC Skin Cell Muscle Cell Nerve Cell

ON__________ ON___________ ON___________ ON_____________

OFF__________ OFF___________ OFF___________ OFF____________

•A—T—G—T—A—A—G—T—A—G—C—C—T—A—A—T—C—A—T—C—G

““Coding” vs. “Non-Coding” vs. “Non-Coding” DNACoding” DNA• “Coding” DNA:• Genes: Nucleotide segments that code

for proteins or polypeptides• Makes up only 3% of the 3 billion base

pairs in the human genome.•  • “Non-Coding” DNA:• Nucleotide segments that do not code

for proteins or polypeptides• Makes up 97% of the human genome.

Composed of regulatory regions, repeating sequences, and other sequences. The functions of many of these regions has not been determined.

Though most of the genome is non-coding, the Though most of the genome is non-coding, the coding regions (“_coding regions (“_genesgenes__”) are what primarily __”) are what primarily determine the __determine the __traitstraits or characteristicsor characteristics____ ____ of organisms.of organisms.