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 groupNitrogenous base (adenine, guanine, cytosine,
or thymine)
All the genetic information for an organism is carried on its DNA.
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.
Structure of DNA Cont’d
Chargaff’s Rules: The four bases (adenine, guanine,
thymine, cytosine) pair up in a certain way
A – TC – 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 ReplicationBefore 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.
RNARNA 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 RNAMessenger 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
TranscriptionRNA 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 CodeThe “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 nucleotidesSubstitutions InsertionsDeletions
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 AATmRNA: AUG CGU ACC UUA
Amino Acids: Met-Arg-Thr-Leu
SUBSTITUTION
DNA: TAC GTA TGG AATmRNA: AUG CAU ACC UUA
Amino Acids: Met-His-Thr-Leu
DNA: TAC GCA TGG AATmRNA: AUG CGU ACC UUA
Amino Acids: Met-Arg-Thr-Leu
INSERTION
DNA: TAT CGC ATG GAA TmRNA: AUA GCG UAC CUU
A
Amino Acids: Ile-Ala-Tyr-Leu