Download DNA and RNA - Midway ISD

Chapter 12
DNA and RNA
Discovery of DNA
How do genes work?
Several scientists from 1928-1954 began
investigating the chemical nature of genes.
DNA was determined to be the molecule
responsible for coding all genes.
What is DNA?
Deoxyribonucleic acid (DNA) is a long
molecule made up of units called
nucleotides.
Nucleotides are made up of:
5-carbon sugar (deoxyribose)
Phosphate group
Nitrogenous base (adenine, guanine, cytosine, or
thymine)
All the genetic information for an
organism is carried on its DNA.
Nucleotides Form DNA
Nucleotide Structure
Structure of DNA
DNA wounds around itself, forming a
twisted ladder (double helix)
Discovered in 1950s by Rosalind Franklin
using X-ray diffraction.
“Double Helix” term coined by Watson and
Crick in the 1950s as well.
Groups of Bases
Purines (larger)
Adenine
Guanine
Pyrimidines (smaller)
Cytosine
Thymine
Structure of DNA Cont’d
Chargaff’s Rules:
The four bases (adenine, guanine, thymine,
cytosine) pair up in a certain way
A – T
C – G
The bases are held together by hydrogen
bonds (hydrogen bonds are weak bonds)
Storage of DNA
Prokaryotes – DNA is found in the
cytoplasm
Most have single circular DNA molecule
Eukaryotes – Much more DNA than in
prokaryotes (up to 1000x as much!)
Condensed into chromosomes
Chromosome number varies wildly from one
species to the next. (humans have 46)
DNA Replication
Before a cell divides, it must copy its
DNA in a process called replication.
During DNA replication, DNA separates
into two strands. DNA polymerase
makes complimentary strands using base
pairing.
Each strand of the original DNA
molecule is used as a template for the
new DNA.
•The area
where DNA
unwinds is
called a
replication
fork.
RNA
RNA is ribonucleic acid.
Like DNA:
Made up of nucleotides
Codes for proteins
Unlike DNA:
Nucleotides are made of ribose (instead of
deoxyribose)
RNA is single-stranded (DNA is double)
The base Thymine is replaced with Uracil in
RNA
3 Types of RNA
Messenger RNA (mRNA)
RNA that carries copies of DNA instructions
Ribosomal RNA (rRNA)
Make up ribosomes (along with proteins)
Transfer RNA (tRNA)
Transfers or carries each amino acid to the
ribosomes as coded for by the mRNA
Transcription
RNA molecules are produced by copying
part of the nucleotide sequence of DNA
into a complementary sequence of RNA
DNA strands are separated, RNA
polymerase uses one strand of DNA as a
template and assembles RNA
Uses base pairing to assemble RNA
A – U (instead of T)
C - G
Genetic Code
The “language” of mRNA is sometimes
called the genetic code.
The genetic code is read 3 letters (or
bases) at a time, called codons.
A codon is made up of 3 nucleotides
that specify for a single amino acid
Amino acids are strung together to
form proteins (polypeptides)
Genetic Code, cont.
 Ex:
UCGCACGGU
Would be read 3 bases at a
time…
UCG-CAC-GGU
Using the code →
Serine-Histadine-Glycine
Start and Stop Codons
AUG codon codes for the assembly of
amino acids to begin. It also codes for
methionine
Three codons code for the assembly of
amino acids to stop – UAA, UAG, and
UGA
Protein Synthesis (Translation)
 Proteins are assembled on ribosomes using
information from mRNA
 Steps:
1 – mRNA transcribed
2 – mRNA attaches to ribosome. tRNA matches
anticodons to codons and brings in amino acids.
3 – amino acids are joined together by peptide
bonds. Free tRNA molecules float away.
4 – polypeptide chain grows until stop codon.
12-4 Mutations
Gene mutations:
Point mutations – involve changes in one
or a few nucleotides
Substitutions
Insertions
Deletions
Frameshift mutations – shift the “reading
frame” of the genetic message
Ex:
AUG UGG CCU UAC → AUG UGG ACC UUA C
Point Mutations
DNA: TAC GCA TGG AAT
mRNA: AUG CGU ACC UUA
DNA: TAC GCA TGG AAT
mRNA: AUG CGU ACC UUA
Amino Acids: Met-Arg-Thr-Leu
Amino Acids: Met-Arg-Thr-Leu
SUBSTITUTION
INSERTION
DNA: TAC GTA TGG AAT
mRNA: AUG CAU ACC UUA
DNA: TAT CGC ATG GAA T
mRNA: AUA GCG UAC CUU A
Amino Acids: Met-His-Thr-Leu
Amino Acids: Ile-Ala-Tyr-Leu
Chromosomal Mutations
 Deletions
 Duplications
 Inversions
Translocations