Chapter 14. Ricin (found in castor-oil plant used in plastics, paints, cosmetics) is toxic because...
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Transcript of Chapter 14. Ricin (found in castor-oil plant used in plastics, paints, cosmetics) is toxic because...
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