Protein Synthesis Lecture Powerpoint

7/29/2019 Protein Synthesis Lecture Powerpoint

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From DNA to Proteins



Nucleic Acids

• Nucleic acids are a type of macromoleculemade from repeating units called

nucleotides.• Each nucleotide is made of the same basic

parts:

– A sugar.

– A base that contains nitrogen.

– A phosphate (PO4) ion.



DNA and RNA

• Each molecule hasthe same basic

structure, except the5-carbon sugar. – Ribose in RNA.

– Deoxyribose in DNA.

• RNA is also usuallysingle-stranded.



Nitrogenous Bases

• There are five different types of nitrogenous bases that make up DNA

and RNA. – DNA: Thymine (T), Adenine (A), Cytosine

(C), and Guanine (G)

–

RNA: Uracil (U) instead of Thymine (T).• The sequence of these bases codes for

all of the proteins produced by the

body.



DNA

• When a cell needs to produce a newprotein, only one specific piece of DNA

from one chromosome is involved.• The human genome has 23 pairs of

chromosomes.



Insulin

• A total of 110 amino acids,coded by 330 base pairs,builds insulin.

• The gene for insulin islocated on chromosome 11.

• The DNA of chromosome 11contains about 134 million

base pairs. – 0.00025% of this chromosome

codes for insulin.



Transcription

• Takes place in the nucleus.

• A messenger RNA molecule (mRNA)is copied from DNA by unzipping aportion of the DNA helix thatcorresponds to a gene

• One side of the DNA strand istranscribed into the appropriatesequence of A, U, C, & G.



Transcription



The Genetic Code

• Base pairs are translated in sets of three,called codons.

• Codons are like the “words” of the DNA“language.”

• One codon corresponds to one amino

acid (the building blocks of proteins).• Examples: – UUU = phenylalanine

– GGA = glycine

– AAA = lycine



Cracking the Code• The genetic code is redundant but not

ambiguous;

– Not ambiguous meaning that each codon

specifies only one amino acid.

• Example: UUU always codes for phenylalanine.

– Redundant meaning that more than one

codon may signals a specific amino acid.• Example: Serine is coded by UCA, UCG, UCC, and

UCU.



Codon chart

•



Translation

• Takes place in the cytoplasm(ribosomes in the rough E.R.)

• Ribosomes read the codons of themRNA and instruct tRNAs (withmatching anticodons) to bring in specificamino acids to be added to the growingpolypeptide chain (protein).



LE 17-4

DNA

molecule

Gene 1

Gene 2

Gene 3

DNA strand

(template)

3

TRANSCRIPTION

Codon

mRNA

TRANSLATION

Protein

Amino acid

3 5

5



Insulin• The hormone insulin is

made of two protein chains,a connecting peptide, and a

signal peptide. – Signal peptides tell the

endoplasmic reticulum todirect the protein outside of

the cell. – Connecting peptides keep the

two chains inactive until theinsulin is actually needed.



How Insulin is Built

1. The double helix of therelevant section of chromosome 11 is unzipped.

2. The mRNA molecule istranscribed using one of theDNA strands as a template.

3. The mRNA molecule is sentoutside the nucleus to therough endoplasmic reticulum.




4. The mRNA is received bya ribosome in the rough

E.R.5. The ribosome starts

translating the mRNA

codons into amino acids.6. The protein is built, one

amino acid at a time,

using tRNA molecules.




• The first 24 amino acids are a signal peptide,telling the endoplasmic reticulum not to release thehormone directly into the cytoplasm.

• The next 30 amino acids make the first part of theinsulin protein.

• The following 31 amino acids make a connectingpeptide. This keeps the two parts of insulin

separate until it is ready to be activated.

• The last 21 amino acids form the second part of theinsulin protein.




• The insulin is sent to the Golgi, where itis packaged.

• The insulin will be released by thepancreas when blood glucose levels aretoo high.



Evolution of the Genetic Code

• The genetic code is nearly universal, shared by thesimplest bacteria to the most complex animals

• Genes can be transcribed and translated afterbeing transplanted from one species to another.

– Most medicinal insulin produced now comes

from cultures of bacteria.



Mutations

• Mutations are changes in the geneticmaterial of a cell or virus

•

Point mutations are chemical changes in just one base pair of a gene.

– This may change a single amino acid in theprotein sequence, which could alter the entire

shape of the protein.

LE 17 23



LE 17-23

Wild-type hemoglobin DNA

mRNA

3 5 5 3

5 3 3 5

Mutant hemoglobin DNA

mRNA

Normal hemoglobin Sickle-cell hemoglobin



Types of Point Mutations

• Point mutations within a gene can bedivided into two general categories

– Base-pair substitutions

– Base-pair insertions or deletions



Substitutions

• A base-pairsubstitution

replaces onenucleotide andits partner with

another pair of nucleotides



Insertions and Deletions

• Insertions and deletions are additions orlosses of nucleotide pairs in a gene

•

These mutations have a disastrous effect onthe resulting protein more often thansubstitutions do

•

Insertion or deletion of nucleotides mayalter the reading frame, producing aframeshift mutation

LE 17-25



LE 17 25

Base-pair insertion or deletion

Frameshift causing immediate nonsense

Extra U

Missing

Frameshift causing

extensive missense

Insertion or deletion of 3 nucleotides:

no frameshift but extra or missing amino acid

Missing

Stop

Stop

Amino end Carboxyl end

Stop

Wild type

mRNA

Protein

5 3



Mutagens

• Spontaneous mutations can occur duringDNA replication, recombination, or repair

• Mutagens are physical or chemical agentsthat can cause mutations.

– Examples include nuclear radiation, UV light, X-

rays, chemicals in tobacco smoke.

Protein Synthesis Lecture Powerpoint

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