Chapter 12 : DNA and RNA

What does DNA look like?

What are the elements that makeup DNA?

What is DNA?

What is DNA?Discovered by Watson & Crick in 1953A long molecule made up of units, called nucleotidesGenes are made of DNADescribed as a twisted ladder, or a double helix

Made up of nucleotides Each nucleotide has 3 parts

* a sugar called deoxyribose* a phosphate group* a nitrogen base

There are 4 nitrogen bases:purines - adenine, guaninepyrimidines - cytosine, thymine

* Every purine pairs with a pyrimidine in order to make a DNA chain

DNA Structure

Purines Pyrimidines

Adenine Guanine Cytosine Thymine

Phosphate group Deoxyribose

Figure 12–5 DNA Nucleotides

•Twisted double helix made of two strands

•Each strand of the helix is a chain of nucleotides

•Held together by hydrogen bonds

Always Together….Great Couple

A & T G & C

•Every nucleotide is paired with a another from the opposite strand.

•Each pair is specific (Chargaff’s Rule).

•Adenine and Thymine pair together

•Guanine and Cytosine pair together

Base Pairing of Nucleotides

Hydrogen bonds

Nucleotide

Sugar-phosphate backbone

Key

Adenine (A)

Thymine (T)

Cytosine (C)

Guanine (G)

Figure 12–7 Structure of DNA

How is DNA organized in a chromosome?

• A nucleus of ONE human cell has more than 1 meter of DNA!!!

•Chromatin: a substance consisting of DNA tightly coiled around proteins called histones.

•The DNA and histones together are called nucleosomes (bead-like structure)

Figure 12-10 Chromosome Structure of Eukaryotes

Chromosome

Supercoils

Coils

Nucleosome

Histones

DNA

double

helix

Chromosome

E. coli bacterium

Bases on the chromosome

Prokaryotic Chromosome Structure

Section 12-2

How can DNA use its double-stranded structure to its advantage for replication???

DNA Replication (Refer to Figure 12-11)When does this occur in the cell cycle?

1) Enzymes un-twist and unzip the molecule (break H bonds between base pairs).

2) Each strand serves as a template

3) Free nitrogen bases form bonds and make complementary strands which follow the base pairing rules.

Template

Figure 12–11 DNA Replication

Growth

Growth

Replication fork

DNA polymerase

New strand

Original strand DNA

polymerase

Nitrogenous bases

Replication fork

Original strand

New strand

DNA vs. RNA

RNA – also a long chain of nucleotides (5-carbon sugar, phosphate group, nitrogenous base)

Differences:

1. RNA sugar = ribose, instead of deoxyribose

2. RNA – usually single-stranded

3. RNA does not have thymine. 1. Uracil instead.

2. Adenine and Uracil pair

RNA is in charge of assembling Amino Acids into Proteins

Transcription: a sequence of DNA is copied into an RNA strand Transcribe the DNA molecule below:

TAATAGCGCATTACGATTATCG

AUUAUCGCGUAAUGCUAAUAGC

•RNA will only start and stop at specific regions of the DNA called promoters.

•We need codons for Protein Synthesis.

•They are directions to make proteins

•Every set of directions tells you where to START and where to STOP (start and stop codons)

•AUG: START UAA, UAG, UGA: STOP

•Proteins are made by joining amino acids into long chains (POLYPEPTIDES)

•Each polypeptide consists of a combo of any or all 20 amino acids.

•Amino acids contain 3 nucleotides. The 3 nucleotides are read as a code called a codon.

•A CODON specifies a single AA that is added to the polypeptide.

Translation Explained

tRNA UAC mRNA AUGCGCAUAACGCAU

Start Codon

Anticodon

methionine

Figure 12–17 The Genetic Code

Translation Practice

Make a polypeptide (chain of amino acids) chain from the mRNA molecule

AUGAUCGCGUAUUGCUACUAG - mRNA

methionine-isoleucine-alanine-tyrosine-cysteine-tyrosine STOP

Mutations - changes in the DNA sequence Gene mutation- changes in a single gene Chromosomal mutation changes in the

entire chromosome (containing many genes)1) Point Mutations - substitution of one nucleotide for

another

2) Frame Shift Mutations - shifting of the genetic code due to insertion or deletion of nucleotide

Mutation AnalogyTHE FAT CAT ATE THE RAT

substitution THE FAT CAT ATE THE CAT *The letter “C” was substituted for the “R”

insertion THE FAT CAT ATE THE RAT THC EFA TCA TAT ETH ERA T

*Because the “C” was added, all other letters shifted down, thereby changing the amino acids that are made.

C

Deletion THE F T CAT ATE THE RAT

THE FTC ATA TET HER AT*Again, the amino acids will change b/c the “F” was removed

A

Mutation Practice

What will the new amino acid be if the 5th nucleotide is substituted with an adenine?

AUGA CGCGUAUUGCUACUAG - mRNAU

What will the new amino acid sequence be if a guanine is inserted between the 9th and 10th nucleotide ?

ASPARAGINE

G

GUA = VALINE

Putting it all together

What is the amino acid sequence that forms from the following DNA molecule? (DNA synthesis)

TACTACACCGTATAACAGGGCCTAGCAACT

Template

ATGATGTGGCATATTGTCCCGGATCGTTGA

(Transcription)DNA - TACTACACCGTATAACAGGGCCTAGCAACT

mRNA - AUGAUGUGGCAUAUUGUCCCGGAUCGUUGA

(Translation)

amino acid sequence

methionine-methionine-tryptophan-histidine-isoleucine-valine-proline-aspartic acid-arginine-stop