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DNA Chapter 8 • Every cell in your body has its own DNA that is identical to every other cell in your body • Your DNA is different from everyone else’s DNA, unless you have an identical twin • DNA is found inside the nucleus of every cell that has a nucleus • If a cell does not have a nucleus, DNA is found floating around in the cytoplasm • What we have been calling genes are really DNA • Chromosomes are made up of DNA • DNA is short for deoxyribonucleic acid • DNA is a molecule that makes up genes and determines the traits of all living things • ALL living things contain DNA • DNA is made up of units called nucleotides • A nucleotide consists of a nitrogenous base, a phosphate group and a 5 carbon sugar called deoxyribose (hence the name deoxyribonucleic acid) • DNA looks very much like a twisted ladder • A ladder has two sides and many rungs • The sides of DNA are made up of two different chemicals – a sugar (deoxyribose) and an acid (phosphate group) • The sugar and acid alternate on each side of the DNA • The rungs of the DNA ladder are made up of chemicals called nitrogen bases • There are four different nitrogen bases in DNA • The 4 bases are adenine, thymine, guanine & cytosine • Adenine only fits with Thymine • Cytosine only fits with Guanine • There are only two bases on each DNA rung • Adenine (A) & Guanine (G) are Purines • Cytosine (C) & Thymine (T) are Pyrimidines • A Purine is always paired with a Pyrimidine • Purines are bigger than Pyrimidines • Between each base pair are hydrogen bonds which hold the two bases together • The ladder of DNA is twisted into a spiral, or helix • DNA is called a double helix • It is called a double helix because it is made up of two strands that are twisted • Scientists did not know much about DNA until the 1900’s • Many different experiments provided info about DNA (8.1, pg 226-228) • The order of nitrogen bases in DNA is the code for all your traits • If the order is changed in any way, there will be a different trait • This works much the same way as the Alphabet • The word pen means one thing, but if we change the E to an I, it means something totally different • If we change the P to an H, it means something else • However, if we change the E in pen to a D, the word would make no sense at all • It is the same with DNA – sometimes changes make the DNA say something else, & sometimes it makes no sense • When DNA is changed to make no sense at all, it could mean death or a severe illness • The difference is there are only 4 letters in the DNA alphabet – but the words are much longer Question 1 •If one strand of DNA is: AT G G CAT G C What is the other strand going to look like? Question 2 •If one strand is: G G C TAGAGAT What is the other strand? DNA & Chromosomes • Prokaryotic cells lack a nucleus, so the DNA is located in the cytoplasm • Prokaryotes have one circular “chromosome” that contains most or all of the cells DNA • Eukaryotic cells have at least 1000 times as much DNA as prokaryotic cells • Eukaryotic DNA is not free in the cytoplasm, but is in the nucleus • The number of chromosomes varies in eukaryotes Size & Length of DNA • DNA molecules are VERY long • E. coli DNA is 4,639,221 base pairs long = approx. 1.6mm • An E. coli cell is only 1.6mm! • The DNA must fit in a space one one-thousandth of its length! Chromosome Structure • The DNA must be folded numerous times to fit into the cell’s nucleus • A human cell contains almost 1000 times as many base pairs as E. coli – human DNA must be folded even more! • Eukaryotic chromosomes contain protein & DNA (form a substance called chromatin) • The DNA is tightly coiled around proteins called histones • The DNA & histones form a nucleosome • Nucleosomes coil together to form a thick fiber and supercoils • The supercoils form a chromosome • We saw chromosomes in Chapter 5 (figure 5.5) DNA Replication • DNA must be replicated before mitosis (and meiosis) • How does that occur? • Each strand of DNA has the info to make the other strand (B/C of base pairing) • During DNA replication, the DNA molecule separates into two strands, then produces two new complimentary strands following the rules of base pairing • In eukaryotes, DNA replication occurs in numerous places on the DNA • The place where the DNA is opening and replication is occurring is called the replication fork • An important enzyme that helps in DNA replication is called DNA Polymerase • There are other enzymes that “unzip” the DNA and then zip the new strands back up again RNA & Protein Synthesis Chapter 8.4 & 8.5 How does DNA code for traits? • DNA must remain in the nucleus – it is too fragile, & too important to be in the cytoplasm • DNA codes for proteins, but proteins are made in the cytoplasm • How can this be? • DNA has a helper molecule to leave the nucleus and go into the cytoplasm to make proteins • This molecule is ribonucleic acid – or RNA RNA Structure • RNA is similar to DNA in that it is made of nucleotides • There is no Thymine in RNA – instead there is a base called Uracil (U), which is similar to thymine RNA Structure • RNA is single stranded, not double stranded (not a double helix) • The sugar in RNA is ribose, not deoxyribose RNA Structure • You can think of RNA as a disposable copy of DNA or a working copy of a single gene • One gene on DNA can make hundreds or thousands of RNA copies of that gene, which can in turn make that many proteins Types of RNA • There are 3 types of RNA, each with a specific structure and function • Messenger RNA (mRNA) is the copy of the DNA • Ribosomal RNA (rRNA) makes up the ribosomes Types of RNA •Transfer RNA (tRNA) is used in the making of proteins and brings the amino acids to the ribosome Making Proteins • There are two main stages of making proteins – Transcription & Translation • Transcription is going from DNA to RNA • Translation is going from RNA to protein Transcription • Transcription occurs in the nucleus • RNA polymerase is an enzyme that is needed, similar to DNA polymerase • Only one strand of DNA is used to make RNA Transcription • Transcription is similar to DNA replication, but the result is single stranded with Uracils in the place of Thymines • The resulting RNA will be complimentary to the DNA strand used Transcription • RNA polymerase knows where genes start because of pieces of DNA called promoters • Promoters have specific base sequences – they act as signals to the RNA polymerase Transcription • Similar pieces of DNA act as signals for the RNA polymerase to stop transcription • The RNA made from transcription needs to be edited before use Transcription • Large pieces of RNA will be cut out, called introns • The remaining pieces of RNA are called exons and are spliced back together with enzymes The Genetic Code • Proteins are made by joining amino acids together • How can an mRNA molecule make a protein if RNA is made of nucleotides? The Genetic Code • The genetic code has only four letters (A,U,G,C) • But, each word in the code is three letters long • Each three letter word makes a specific amino acid The Genetic Code • The 3-letter words are called codons • For instance, the codon AUG always codes for the amino acid methionine The Genetic Code • The mRNA sequence AUGGCGUGU can be broken up into its codons • AUG – GCG – UGU • The amino acid sequence is: –methionine, alanine, cysteine The Genetic Code • Figure 8.13 shows the genetic code (page 244) • Here are some other Genetic Code charts • (You will have this on a test) Translation •Translation occurs outside of the nucleus, in the cytoplasm of the cell, on the Ribosome Translation • Translation requires the 3 types of RNA • mRNA = messenger RNA • tRNA = transfer RNA • rRNA = ribosomal RNA Translation • Attached to each tRNA is an Amino Acid • Amino acids make up the polypeptides • There is also an anti-codon on the tRNA Translation • The anti-codon on the tRNA matches up with the codon on the mRNA Translation – Step by Step • First, the mRNA must be transcribed from the DNA, leave the nucleus & go to the cytoplasm • Then, the mRNA attaches to a ribosome in the cytoplasm Translation – Step by Step • The mRNA moves through the ribosome in one direction & for each codon, the proper amino acid is attached to the polypeptide chain via the tRNA • The anti-codon on the tRNA matches up with the codon on the mRNA Translation – Step by Step • For example – the start codon on the mRNA is A U G • The anti-codon is U A C • Then, a second tRNA with the next amino acid attaches to a different site on the ribosome Translation – Step by Step • The ribosome forms a peptide bond between the 1st and 2nd amino acid st • The bond between the 1 tRNA and its amino acid is broken • The ribosome then moves to the rd 3 codon Translation – Step by Step • The polypeptide continues to grow in this fashion until the ribosome reaches a stop codon (UAA, UAG or UGA) • The mRNA molecule is then released & the polypeptide is released into the cytoplasm Genes and Proteins • The polypeptide made in translation will be modified by the golgi or the rough ER and made into a fully functional protein • Proteins control almost everything that living cells do! Genes and Proteins • Why is it important that the mRNA is read in one direction only? • Decode the following DNA strand from left to right GACAAGTCCACAATC Genes and Proteins • The mRNA strand is: CUGUUCAGGUGUUAG • The amino acid sequence is leucine-phenylalaninearginine-cysteine-stop Genes and Proteins • Now, read the mRNA strand right to left GAUUGUGGACUUGUC • The amino acid sequence is aspartic acid-cysteineglycine-leucine-valine Genes and Proteins • The resulting proteins are completely different and will do different things – maybe even not function at all! • Next, we’ll talk about mutations and how they affect proteins and traits Mutations • Every now and then cells make a mistake in copying DNA or making RNA • These mistakes are called mutations • Mutations come in all “shapes & sizes” Mutations • Gene mutations result from changes in a single gene • Chromosomal mutations involve changes in a whole chromosome Mutations • Most gene mutations involve just one nucleotide, but some involve many • Mutations that affect only one nucleotide are called point mutations Mutations • When one nucleotide is switched out for another, it will generally change only one amino acid in the protein • If one nucleotide is inserted or deleted, more severe changes occur Mutations • Mutations that add or delete a nucleotide are called frame-shift mutations • They affect every amino acid after the mutation – very bad • HOW??? Example - substitution • DNA: TCT ACA ACC ACG • RNA: ?? • Amino Acid: ?? • New DNA: TCTTCAACCACG • NEW RNA: ?? • New Amino Acids: ?? Example - substitution • DNA: TCT ACA ACC ACG • RNA: AGA UGU UGG UGC • Amino Acid: arginine – cysteine – tryptophan – cysteine NEW • DNA: TCT TCA ACC ACG • RNA: AGA AGU UGG UGC • Amino Acids: arginine – serine – tryptophan – cysteine Example - Deletion • DNA: TTCGTCATGCACATC • RNA = ?? • Amino Acids = ?? • New DNA: TTCGTCTGCACATC • New RNA = ?? • New Amino Acids = ?? Example - Deletion • DNA: TTCGTCATGCACATC • RNA: AAG CAG UAC GUG UAG • Amino Acids: lysine-glutaminetyrosine-valine-stop NEW • DNA: TTCGTCTGCACATC • RNA: AAG CAG ACG UGU AG • Amino Acids: lysine-glutaminethreonine-cysteine-… Chromosomal Mutations • There are different types of chromosomal mutations • A deletion is when part of the chromosome is deleted • A duplication is when a part pf the chromosome is duplicated Chromosomal Mutations • An inversion is when parts of a chromosome are switched around • A translocation is when pieces of one chromosome end up on a different chromosome