Chapter 12DNA 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 groupNitrogenous 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 BasesPurines (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 – 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.

•The area where DNA unwinds is called a replication fork.

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

Chromosomal Mutations

Deletions

Duplications

Inversions

Translocations