Chapter 12DNA and RNA

What is DNA again?Deoxyribonucleic acidLong double-stranded molecule of

nucleotidesStores genetic code that is transferred

to future generations and cellsCodes for synthesis of proteins

What is a nucleotideA nucleotide has 3 parts:

◦A sugar (deoxyribose for DNA)◦A phosphate group◦A nitrogen base

Nitrogen base pairingThe order of nitrogen bases are what

determines your genetic codeThe order of the bases determines the

proteins that are made4 different bases exist:

◦Purines: Guanine, Adenine Grade A Pure

◦Pyrimidines: Cytosine, ThyminePairing is also called Chargaff’s Rule

Chargaff’s Rule

Result: A=T, G=C

But what is the structure?In the early 1950s Rosalind Franklin

used X-ray diffraction took a “picture” of DNA◦Difficult to tell what the structure is

from the image◦Strands twisted around one another

But what is the structure?James Watson and Francis Crick used

Franklin’s images and their models to determine the correct structure: double helix◦Two strands wound around one

another◦“Twisted ladder”◦Later discovered that hydrogen

bonds hold the two sides of the ladder together

◦Can only from between adenine and thymine, or guanine and cytosine (A=T, G=C)

◦The base pairing explained Chargaff’s Rule

DNA Structure

DNA Structure DNA is very long; E. coli’s contains

4,639,221 base pairs (length =1.6 mm)◦Must fit in an organism 1/1000 its

length◦Must be packed very tightly to fit

Human cell contains almost 100 times the base pairs◦Chromatin consists of DNA packed

around proteins called histones◦These compact together during

mitosis to create our visible chromosomes

DNA Structure

DNA replicationRemember that before a cell divides it

has to copy its DNA◦If we separate each strand, we can

use it to make a copy of the other◦If we have an A, we match it with a

T; if a G, we match it with a C (called complementary)

In prokaryotes, replication begins at one point and proceeds typically in both directions

In contrast, eukaryotes begin replication in hundreds of locations (due to its longer length)

DNA Replication

DNA replicationTwo replication forks formed, each

strand has a new complementary strand added◦TACGTT ATGCAA◦Results in two identical DNA

molecules◦One strand is new, the other old

DNA was unzipped (breaking of hydrogen bonds)◦DNA polymerase (enzyme) joins

individual nucleotides to produce a DNA molecule

What is RNA?Ribonucleic acidIntermediate molecule between DNA

and proteinsDiffers from DNA in 3 ways:

◦Single stranded (vs. double)◦Ribose instead of deoxyribose◦Utilizes uracil instead of thymine (A-

U, G-C)

Type of RNAThere are 3 types of RNA:

◦messenger RNA (mRNA)- carries copies of the protein instructions

◦ribosomal RNA (rRNA)- makes up the ribosome, along with several proteins

◦transfer RNA (tRNA)- take the appropriate (base pairing anti-codon to codon) amino acid to the ribosome

Types of RNA

Protein synthesis: how we make proteins?Two processes or steps:

◦Transcription- mRNA is made from the DNA and travels to the cytoplasm to find a ribosome

◦Translation- tRNA brings to the ribosome the amino acids to build the primary structure of a protein and the result is a free polypeptide that will then fold up into the shape of the protein

Remember: amino acids are held together by peptide bonds

Transcription

TranscriptionRNA polymerase binds to DNA and

separates the strandsRNA polymerase uses one strand to

serve as a template to create a strand of RNA

RNA polymerase only binds to regions of DNA called promoters (landing pad)

The RNA is next edited, removing the introns, while leaving the exons (exons=expressed)

RNA Splicing

TranslationProteins are made by joining amino acids

(1 of 20) into long chains (polypeptides)◦How do 4 letters lead to so many

different amino acids?◦Read 3 letters at a time, called a codon◦UCGCACGGU◦UCG-CAC-GCU◦Serine-Histidine-Glycine◦Some codons code for the same amino

acid◦AUG is the start codon

TranslationCodon – 3 nucleotides that code for an

amino acid. Found on the mRNA.Anticodon - Fit the codon. Found on

the tRNA

DNA: master plan; never brought to work site

mRNA: blue prints made from master plan

Ribosomes: building sitetRNA: trucks unloading materials

Codons

Codons

Translation

Transcription and Translation

MutationsMutations – changes made in the genetic

material; can occur during copyingPoint mutation – effect only one

nucleotide. Frameshift mutation – an insertion or

deletion that effects the remaining string of nucleotides.

Chromosomal mutations – changes in the number or structure of chromosomes.

Polyploidy – when an organism has extra sets of chromosomes.

MutationsTHE FAT CAT ATE THE RAT

◦Delete H (Frameshift / deletion)TEF ATC ATA TET HER AT

◦Change H to L (Point mutation)TLE FAT CAT ATE THE RAT

MutationsDNA: TAC GCA TGG AATRNA: AUG CGU ACC UUAAA: Met Arg Thr Leu

◦SubstitutionDNA: TAC GTA TGG AATRNA: AUG CAU ACC UUAAA: Met His Thr Leu

◦ InsertionDNA: TAT CGC ATG GAA TRNA: AUA GCG UAC CUU AAA: Ile Ala Tyr Leu