From DNA to proteins

BioH Chapter 13

The BIG picture – Gametes to genes

DNA - RNA comparison

DNA• Nucleotide (P-S-B)

• Deoxyribose• A,T,C,G• Nucleus only

• Carries/transfers genetic info

• Double strand

RNA

• Nucleotide (P-S-B)• Ribose• A,U,C,G• Nucleus AND

cytoplasm• Carries/transfers

genetic info AND PROTEIN SYNTHESIS

• Single strand

• Monomer

•Sugar•Bases used•Usual location

•Function

•Structure

Characteristic

TranscriptionThe process of using a DNA template to make RNA

(Click for video)

Transcription details• Enzymes used

Helicase – unwind & start strand separation RNA polymerase – brings complementary

base-matching nucleotides Ligase – corrections and gap corrections

• Promoter sequence on mRNA - signals “start” for transcribing DNA sequence into RNA sequence

• ONE strand only – forming juvenile RNA

• Uracil used instead of Thymine

• Use Cytosine, Guanine, Adenine (same as DNA)

Juvenile RNA Immature RNA formed by transcription in nucleus(juvenile RNA, pre-mRNA)

Forms mature mRNA at nuclear envelope through use of specific enzymes

Most eukaryotic genes contain base sequences that must be removed before translation can occur (introns). They will never be used to form proteins.

The RNA genes that remain (exons) form specific proteins that determine traits

Starting Translation

NEED solve three issues:

1.Instructions on what specific proteins to build

2.Capture and provide raw material (amino acids) with which to build proteins

3.Place at which to build proteins

mRNA

tRNArRNA

Types of RNAmRNA – messenger RNA – carries protein building instructions (very long)

tRNA – transfer RNA – picks up amino acid components and delivers them to a ribosome to be assembled into proteins (3 bases long)

rRNA – ribosomal RNA – attracts proteins to form a ribosome site for protein synthesis (medium length)

Translation

Process of translating mRNA base sequence into proteins

Providing the message - mRNA

• Contains the coded instructions to make specific proteins (based on the nucleotides’ base sequence)

• 3 bases as a group (triplet) are called a codon

• Use the Genetic Code charts to decipher which amino acids are coded by each codon

Genetic Code charts

Use the mRNA codon (transcribed from DNA) to read the charts

64 codons code for only 20 different amino acid building blocks

(video)

Supplying raw material - tRNA

• tRNA attracts amino acids in cytoplasm based “lock and key” structure

• Matches the codon on the mRNA with its own 3- base anti-codon

• Delivers specific amino acids to functional ribosome to build polypeptide chain (protein)

(Video)

Ribosome Formation - rRNA

rRNA combines with other proteins to form the basis of a functional ribosome

Small subunit Large subunit

(Video)

Finally – TRANSLATION !Three stages: Initiation, Elongation & Termination

Initiation

• Mature mRNA leaves nucleus into cytoplasm through nuclear pores

• Many free amino acids, tRNA and ribosomal subunits floating in cytoplasm

• Initiator tRNA (attached to the amino acid methionine) attaches to small ribosomal subunit, which then attaches to end of mRNA

• mRNA moves through ribosome until reaches “start” codon (AUG) on mRNA

• Large subunit attaches, forming functional ribosome

• Starts next phase = elongation

More translation Elongation

• The initiator tRNA anticodon-mRNA codon bond causes the next bonding site in the ribosome to attract the specific anticodon of another tRNA

• A 2nd tRNA (with its amino acid) bonds with the 2nd m RNA codon

• Once this happens, enzymes help form a peptide bond between the two nearby amino acids

• While other enzymes break the initiator amino acid bond and the first codon-anticodon bond, releasing the intiator tRNA and leaving Methionine to form a peptide bond with the second amino acid

• The 2nd tRNA moves into the 1st ribosomal bonding site• A 3rd tRNA is attracted to the open

ribosomal bonding site, allowing the process to continue

• This process continues until a “STOP” codon is read, initiating the last phase = termination

Translation is terminatedTermination

•The mRNA codon UAA, UAG or AGA (“STOP”) occupies the second ribosomal bonding site•No tRNA anticodon bonds with these codons•This signals the synthesis process to stop•The polypeptide chain (protein) is released from the ribosome•The two ribosomal subunits separate

We have PROTEINS!

NEXT?• Free-floating proteins• Golgi apparatus packaging• Enter ER for transport

Problems – Gene MutationsPoint Mutations – single base-pair change

Substitution – one base substituted for the correct oneMay result in only one amino acid mistake – protein may still function correctly

Frameshift Mutations Insertion – extra base insertedDeletion – base is omittedResults in all codons after mutation to be incorrect and may cause protein to malfunction

Problems – Gene MutationsPoint mutations – single base changes

Deletion – base omitted

Insertion – extra base inserted

Substitution – one base substituted for the correct one

Transposition – DNA segments transposed (exchanged) with another

Mutation causes

Spontaneous

Exposure to mutagens (UV, gamma & X-rays)

Natural & synthetic chemicals

Significance and causes of gene mutations

CausesSpontaneousExposure to mutagens (UV, gamma & X-rays)Natural & synthetic carcinogenic chemicals

ImportanceMany, if not most, mutations are neutral – causing little or no effect on protein functionCan be harmful, causing genetic disorders – cystic fibrosis, sickle cell disease, cancers, HIV toleranceCan be beneficial, evolutionary changes have come about due to positive mutations that allow organisms to better survive their environment

(video)

The whole process