Download DNA RNA

Ch. 10.6-16 Gene  Protein
• A gene is a sequence of nucleotides that code for
a polypeptide (protein)
• Hundreds-thousands of genes are on a typical
chromosome.
The “Central Dogma” of Genetics
• Genes are coded DNA instructions for the
construction of proteins.
• DNA is located in the nucleus, but proteins
are made in ribosomes
• To avoid damage to the DNA molecules, they
are first decoded into RNA which is sent to
the ribosome to be the instructions for
protein synthesis.
DNA  RNA  Protein
DNA v. RNA
DNA
1. Sugar is deoxyribose
2. Double-stranded
3. A, T, C & G bases
RNA
1. Sugar is ribose
2. Single-stranded
3. Uracil instead of
thymine
Three types of RNA
• mRNA (messenger) – carries copies of instructions
for assembling amino acids into proteins
• rRNA (ribosomal) –
makes up part of the ribosome
• tRNA (transfer) – carries each AA
needed to build the protein to the ribosome
The Flow of Genetic Information
Protein synthesis occurs in 2 steps:
transcription (DNA  RNA) & translation (RNA  protein)
Transcription
• RNA is produced when RNA polymerase
copies a sequence of DNA into a
complementary RNA strand.
DNA: TACGGACACATT
RNA: AUGCCUGUGUAA
The Genetic Code
Additional Details of Transcription
1. Initiation: RNA polymerase attached to promoter
sequence of DNA and RNA synthesis begins
2. Elongation: RNA elongates and the synthesized RNA
strand peels away from DNA template allowing the
DNA strands to come back together in regions
transcribed
3. Termination: RNA polymerase reaches sequence of
DNA bases called a terminator signaling the end of the
gene and polymerase molecule detaches
RNA Processing (Editing)
Intron- noncoding DNA
(“junk”)
Exon – coding sequences
Translation
mRNA  Polypeptide
• Decoding of an mRNA message into
a polypeptide chain (protein)
• mRNA molecules are “read” in
three base segments called codons
• Each codon specifies a particular
amino acid
• Some AA are specified by more than
one codon.
A Closer Look at tRNA
A Closer Look at Ribsomes
Steps of translation - Initiation
• After RNA is transcribed in the nucleus, it
enters the cytoplasm and attaches to a small
ribosomal subunit
• special initiator tRNA binds to the start
codon bringing in the amino acid MET
• large ribosomal subunit binds to the small
one creating a functional ribosome
Steps of Translation - Elongation
•
The anticodon of an incoming tRNA molecule with
AA pairs with mRNA codon in A site
•
AA detaches from tRNA in P site and peptide bond
forms between it and the AA in the A site
•
translocation – P site tRNA leaves the ribosome and
the A site complex (AA, tRNA anticodon and mRNA
codon) shift s over to the P site
• Process continues until a STOP codon is reached
Steps of Translation - Termination
• Stop codons – UAA, UAG, and UGA do not code
for amino acids
• These codons signal the end of translation
• The completed polypeptide is released from the
last tRNA and exits the ribosome
• The ribosome splits into individual subunits
Animation
Mutations
• Mutations are changes in the genetic material
– Gene Mutations: change in the nucleotide
sequence within a single gene
• Point mutations are gene mutations involving a
change in one or a few nucleotides
– Substitution: usually changes only one AA
– Frameshift: addition or deletion of a nucleotide
shifts the grouping of codons
Substitutions Mutations
Frameshift Mutations
Chromosomal mutations: change in an entire
chromosome; may involve loss or duplication of
multiple genes
4 Types of Chromosomal Mutations
** Polyploidy = extra set of chromosomes
Causes & Effects of Mutations
• Causes: Mutagenesis can occur in many ways
– Spontaneous mutations occur during DNA replication or
recombination
– Physical or chemical agents called mutagens may induce
mutations (ex. High energy radiation from x-rays or UV
light)
• Effects: Can be harmful, beneficial or neither
– May cause of genetic disorders
– May be beneficial and lead to production of proteins
with new or altered activities, which has an important
role in the evolutionary process of natural selection
– Some mutations are “silent” and have no effect
because the nucleotide change results in a new codon
that codes for the same amino acid as the original
codon