PROTEIN SYNTHESIS AND

ALBINISM

From Genotype to Phenotype• Phenotype or the observed trait of an organism is coded in the sequence of bases

which we call genotype.

• The order of the bases specifies which protein will be made.

• Proteins build the traits we observe in organisms.

• We call this the genetic code because the code is passed from parent to offspring genetically.

• Example: Albinism• Organisms with albinism express the phenotype (un-pigmented skin) because of the

specific sequence of bases carried in their DNA.

Molecules of Protein Synthesis• DNA

• contains bases A, C, T, G• Is found in the nucleus• Is double stranded and shaped like a

double helix• Contains the genetic code for all

proteins that make up an organism.

• mRNA- messenger RNA• Contains bases A, C, U, G• Is made in the nucleus but gets

exported to the cytoplasm• Is single stranded and linear• Carries the message from the nucleus

to the ribosome when a protein needs to be synthesized.

• tRNA- transfer RNA• Small molecule that carries an amino

acid. • Each amino acid is attached to anti

codons • The anticodons base pair with the

mRNA codons.

• Ribosomes• Organelles that synthesize proteins• Read mRNA message and match

them to tRNA

• Amino Acids (aa)• Building blocks of proteins• Carried on tRNA

Protein Synthesis• 1. The code for the protein is

transferred from DNA to mRNA while in the nucleus, using base paring.

• 2. The mRNA is exported from the nucleus to the cytoplasm where it attaches to a ribosome. Remember ribosomes are protein factories!

• 3. tRNA reads the message from the mRNA three letters at a time. The three letters are called a codon. Each tRNA has an anti-codon that lines up with the codon on the mRNA.

• 4. The amino acid that is carried by the tRNA is added to the growing protein.

DNA to mRNA• Base Pairing

• Molecules in the nucleus transfer the genetic code from DNA to mRNA by complementary base pairing. • A pairs with U• T pairs with A• G pairs with C• C pairs with G

• Example: Use base pairing to transfer the DNA code to the mRNA message.

DNA: TAC-TCA- ATG-AGG-GTC-AGC

mRNA:

mRNA to tRNA

• Base pairing• The mRNA codons are read

three letters at ta time and matched to the anti-codons on the tRNA molecules using the following base pairing rules:• A pairs with U• U pairs with A• G pairs with C• C pairs with G

• Example: Use base pairing to transfer the mRNA codons to tRNA anti-codons.

mRNA: AUG-AGU-UCC-CAG-UCG

tRNA:

tRNA to Protein

• Ribosomes read the message from mRNA and use tRNAs to bring the correct amino acid to build the protein.

• Each tRNA is attached to an amino acid.

• Example:

mRNA: AUG-AGU-UCC-CAG-UCG

tRNA:

aa:

Start

Mutations

• Any change in the sequence of bases in DNA is a mutation.

• Mutations cause a cell to produce incorrect proteins during proteins synthesis.

• This results in an abnormal phenotype.

• Sources of Mutation• Errors in DNA replication• Errors during meiosis

• Effects of Mutations• Mutations introduce changes into

organisms and are the ultimate source of genetic variation in organisms.

• If the changes increase the survival of the organism then it is helpful and leads to evolution. (ex: white bear in the artic)

• If the changes decrease the survival of the organism the it is harmful and leads to extinction. (ex: white lemur in the jungle)

Albinism• Albinism is the result of a

mutation that disrupts the synthesis of a protein called melanin.

• Melanin is the protein that is found in skin hair and eyes that gives them dark color.

• Different concentrations of Melanin create the wide array of pigments we see in skin color.

• Albinism is a disorder that an organism is born with (congenital).

• The most common form of albinism in humans is Oculocutaneous albinism type 1 (OCA1) and is recessive.

• OCA1 is caused by a mutation to the TYR gene. It affects 1 in 4000 people .

TYR Mutations

• This gene codes for the protein that is responsible for synthesizing melanin.

• TYR codes for a protein called tyrosinase. This protein is an enzyme which produces melanin.

• When there is a mutation in the TYR gene, protein synthesis transfers that mutation to build a faulty tyrosinase enzyme. This enzyme can not make melanin.

• There are over 450 different mutations in this gene that can cause Albinism. • http://

www.ifpcs.org/albinism/oca1mut.html

• Most of them are the result of a single base mutation.

• One of the more common mutations is called TYR-R299H and involves the mutation of an G to an A at base #896

TYR Protein Synthesis• Normal TYR gene Sequence

• DNA: gcctcggcct• mRNA:• aa: Arg

Normal TYROSINASE= Normal melanin synthesis!

Normal Melanin amounts = pigmented skin!

Normal genotype= normal phenotype!

• OCA1 TYR gene Sequence• DNA: gcctcagcct• mRNA:• aa:His

Faulty TYROSINASE= No melanin synthesis

No Melanin= Albinism

Mutated genotype= mutated phenotype

Cells protected from UV light radiation.