FROM DNA TO PROTEINChapter 14

IMPACTS, ISSUES:

RICIN AND YOUR RIBOSOMES Ricin (found in castor-oil plant used in

plastics, paints, cosmetics) is toxic because it inactivates ribosomes, the organelles which assemble amino acids into proteins, critical to life processes

THE NATURE OF GENETIC INFORMATION

Each strand of DNA consists of a chain of four kinds of nucleotides: A, T, G and C

The sequence of the four bases in the strand is the genetic information

DIFFERENCES BETWEEN DNA AND RNA

Sugar = Deoxyribose Double stranded Bases

Cytosine Guanine Adenine Thymine

Sugar = Ribose Single Stranded Bases

Cytosine Guanine Adenine URACIL (U)

DNA RNA

These chemical differences make it easy for the enzymes in the cell to tell DNA and RNA apart

RIBONUCLEOTIDES AND NUCLEOTIDES

DNA AND RNA

THE 3 MAIN TYPES OF RNA

Messenger RNA (mRNA)Carry a copy of the instructions

from the nucleus to other parts of the cell

Ribosomal RNA (rRNA)Makes up the structure of

ribosomes

Transfer RNA (tRNA)Transfers amino acids (proteins) to

the ribosomes to be assembled

GENE EXPRESSION A cell’s DNA sequence (genes) contains

all the information needed to make the molecules of life

Gene expressionA multistep process including transcription

and translation, by which genetic information encoded by a gene is converted into a structural or functional part of a cell or body

In transcription, a strand of mRNA is assembled on a DNA template using RNA nucleotidesUracil (U) nucleotides pair with A

nucleotidesRNA polymerase adds nucleotides to the

transcript

14.2 TRANSCRIPTION: DNA TO RNA

BASE-PAIRING IN DNA SYNTHESIS AND TRANSCRIPTION

THE PROCESS OF TRANSCRIPTION

RNA polymerase and regulatory proteins attach to a promoter (a specific binding site in DNA close to the start of a gene)

RNA polymerase moves over the gene in a 5' to 3' direction, unwinds the DNA helix, reads the base sequence, and joins free RNA nucleotides into a complementary strand of mRNA

 Transcription

TRANSCRIPTION

Fig. 14-5b, p. 219

TRANSCRIPTION Many RNA polymerases can transcribe a

gene at the same time

POST-TRANSCRIPTIONAL MODIFICATIONS In eukaryotes, RNA is modified before it

leaves the nucleus as a mature mRNA Introns = Nucleotide sequences that

are removed from a new RNA Exons = Sequences that stay in the

RNA

ALTERNATIVE SPLICING Alternative splicing

Allows one gene to encode different proteins

Some exons are removed from RNA and others are spliced together in various combinations

After splicing, transcripts are finished with a modified guanine “cap” at the 5' end and a poly-A tail at the 3' end

MRNA – THE MESSENGER

mRNA carries protein-building information to ribosomes and tRNA for translation

CodonA sequence of three mRNA nucleotides that

codes for a specific amino acidThe order of codons in mRNA determines

the order of amino acids in a polypeptide chain

GENETIC CODE

Genetic code Consists of 64 mRNA

codons (triplets) Some amino acids can

be coded by more than one codon

Some codons signal the start or end of a gene AUG (methionine) is a

start codon UAA, UAG, and UGA are

stop codons

CODONS OF THE GENETIC CODE

RRNA AND TRNA – THE TRANSLATORS tRNAs deliver amino acids to ribosomes

tRNA has an anticodon complementary to an mRNA codon, and a binding site for the amino acid specified by that codon

Ribosomes, which link amino acids into polypeptide chains, consist of two subunits of rRNA and proteins

TRNA

TRANSLATION Translation = mRNA to protein Occurs in the cytoplasm on the

ribosomes Translation occurs in three stages

InitiationElongationTermination

 Translation

INITIATION

An initiation complex is formedA small ribosomal subunit

binds to mRNAThe anticodon of initiator

tRNA base-pairs with the start codon (AUG) of mRNA

A large ribosomal subunit joins the small ribosomal subunit

ELONGATION The ribosome assembles a polypeptide

chain as it moves along the mRNA Initiator tRNA carries methionine, the first

amino acid of the chainThe ribosome joins each amino acid to the

polypeptide chain with a peptide bond

Fig. 14-12d, p. 223

TERMINATION

When the ribosome encounters a stop codon, polypeptide synthesis endsRelease factors bind to the ribosomeEnzymes detach the mRNA and polypeptide

chain from the ribosome

POLYSOMES Many ribosomes may

simultaneously translate the same mRNA, forming polysomes

14.5 MUTATED GENES AND THEIR PROTEIN PRODUCTS If the nucleotide sequence of a gene

changes, it may result in an altered gene product, with harmful effects

MutationsSmall-scale changes in the nucleotide

sequence of a cell’s DNA that alter the genetic code

Tay Sachs – One wrong letter - from PBS Cracking the Code of Life – 9:38 and 57:00

COMMON MUTATIONS Base-pair-substitution

May result in a premature stop codon or a different amino acid in a protein product

Example: sickle-cell anemia Deletion or insertion (frame shift)

Can cause the reading frame of mRNA codons to shift, changing the genetic message

Example: Huntington’s disease

COMMON MUTATIONS

WHAT CAUSES MUTATIONS? Transposable elements

Segments of DNA that can insert themselves anywhere in a chromosomes

Spontaneous mutationsUncorrected errors in DNA replication

Harmful environmental agents Ionizing radiation, UV radiation, chemicals

MCCLINTOCK’S TRANSPOSABLE ELEMENTS - 100 GREATEST DISCOVERIES – START AT 12 MIN

MUTATIONS CAUSED BY RADIATION Ionizing radiation damages

chromosomes, nonionizing (UV) radiation forms thymine dimers

INHERITED MUTATIONS

Mutations in somatic cells (the cells that are not sperm or eggs) of sexually reproducing species are not inherited

Mutations in a germ cell or gamete may be inherited, with evolutionary consequences

SUMMARY: PROTEIN SYNTHESIS IN EUKARYOTIC CELLS