DNA & Protein Synthesis

Chapter 3

DNA• Deoxyribonucleic acid is a nucleic acid

(which is made up of nucleotides=sugar, phosphate and base)

• Made up of two strands that pair together by something called the complimentary rule

• DNA has specific pairing between the nitrogen bases:• ADENINE – THYMINE• CYTOSINE – GUANINE

OO=P-O O

Phosphate Group

NNitrogenous base (A, G, C, or T)

CH2

O

C1C4

C3 C2

5

Sugar(deoxyribose)

Nucleotides

DNA Double Helix

P

P

P

O

O

O

1

23

4

5

5

3

3

5

P

P

PO

O

O

1

2 3

4

5

5

3

5

3

G C

T A

DNA Replication• DNA MUST be copied!!• DNA produces two identical new

complementary strands following the base pairing rules (A-T, G-C)

• Each original strand of DNA serves as a template for each new strand

Replication Step #1

• DNA Polymerase

comes on the scene• UNWINDS the DNA

upstream• UNZIPS the DNA• This site is known

as the replication bubble

Replication Step #2

• Complementary Bases begin

adding into both sides of the DNA• A binds with T, C binds with G (no

other possibility because of the shape of the bases!)

• The DNA Polymerase precedes the paired bases and clips out any that are already added ahead of it

Replication Step #3

• Finally you have 2 identical copies of DNA

• The final job of the Polymerase is to Proofread the nucleotides after they are added and to clip out any that are incorrectly paired

The Code

• DNA molecules carry the code for all the genes of an organism

• Genes are pieces of the DNA molecule that code for specific proteins

• The process of making genes into proteins is called protein synthesis-which occurs OUTSIDE of the nucleus on the ribosome

Steps of Protein Synthesis

• The DNA code of the gene segment must be copied in the nucleus of the cell

• The code must be carried from the nucleus into the cytoplasm and then to the ribosome

• The protein is then assembled from the code and released • These steps are carried about by RNA (Ribonucleic

acid)

Carrying the Code Out

• RNA is a molecule that is used to translate the code from a DNA molecule into a protein

• It is very similar to DNA except: it is single stranded, it’s sugar is ribose and instead of thymine as a base, it uses Uracil (so A pairs with U in RNA only!)

• There are three types of RNA: messenger, ribosomal and transfer (ALL are involved in protein synthesis)

Transcription

• Step one of protein synthesis is the manufacturing of messenger RNA (mRNA)

• This making of the mRNA is called transcription• Transcription begins when a region of the DNA

unwinds and separates (this separated segment is a gene)

• This unwound segment serves as a template for the soon to be mRNA strand

mRNA

• The mRNA strand is assembled from individual RNA nucleotides that are present in the nucleus

• RNA polymerase (an enzyme) picks up these nucleotides and matches them to their DNA complement from the template that has just been unzipped

• At this point, the mRNA separates and leaves the nucleus-moving into the cytoplasm and settling on a ribosome-this is where translation begins

Translation

• Ribosomes are made up of ribosomal RNA (rRNA)• On these ribosomes mRNA is decoded (translated)

and a corresponding polypeptide (amino acids) is formed

• When we “decode” a chain of nucleotides we “read” it in a three base code called a codon• For example: our mRNA sequence could be

AUGACAGAUUAG• The corresponding codon would be AUG ACA GAU UAG

Codons

• The three nucleotide codon has the specific function of corresponding to a particular amino acid

• How does this work?• The mRNA is bound to the surface of the ribosome at

the first nucleotide segment (called a start codon)• The cytoplasm in which the rest floats contains amino

acids and a third kind of RNA, transfer RNA (tRNA)

tRNA

• This molecule contains a three part nucleotide segment called an anticodon (this is the exact match of one mRNA codon)

• The anticodon corresponds exactly to one of the 20 kinds of amino acids

• Once the tRNA binds the amino acid it travels to the ribosome surface and there the three tRNA bases pair with their three complementary mRNA bases

Finishing Up Translation

• The amino acid that is bound to the tRNA is then added to the growing polypeptide chain at the surface of the ribosome

• The ribosome facilitates this process by moving along the mRNA chain until it reaches a stop codon (a three nucleotide segment that tells the ribosome that the translation is complete)

• The ribosome then releases the newly-formed polypeptide chain which moves out into the cell as a fully functioning protein