DNA & GenesChapter 11

Discovery of DNAThe composition of DNA was first

described correctly in 1952 by two scientists:1. Watson2. Crick

They discovered that it was formed from two long chains of nucleotides shaped much like a spiraling ladder.

They called this shape the double helix.

DNA: The Molecule of HeredityDeoxyribonucleic Acid (DNA) is a very

long molecule that contains the information of life.

DNA is a polymer made of repeating subunits called nucleotides.

A nucleotide has three parts:A simple sugarA phosphate groupA nitrogen base

Structure of DNASimple sugar

The sugar in DNA is deoxyriboseIt gives DNA its name

Phosphate groupComposed of one atom of phosphorus

surrounded by four oxygen atoms

Structure of DNANitrogen Base

A carbon ring structure that contains one or more atoms of nitrogen

There are four possible nitrogen bases:adenine (A)guanine (G)cytosine (C)thymine (T)

Nitrogen Bases

Purine – double ring nitrogen basesAdenine (A)Guanine (G)

Pyrimidine – smaller, single-ring nitrogen bases Cytosine (C)Thymine (T)

Draw these structures in your notes – Page 288

Base-Pairing RulesThe two sides of the double helix are

connected together by hydrogen bonds.

Hydrogen bonds form between two nitrogen bases that make up the rungs of the ladder.

The two nitrogen bases on the same rung of the DNA ladder are referred to as a base pair.

Base-Pairing Rules for DNAGuanine always pairs with Cytosine

Adenine always pairs with Thymine

The strictness of base-pairing results in two strands that are complementary. This means the sequence of bases on one strand determines the sequence of bases on the other strand.

Example: T C G A A T T G C

__ __ __ __ __ __ __ __ __

Base-Pairing in DNAAll organisms contain chromosomes composed of

DNA made up of nucleotides with guanine, cytosine, adenine, and thymine.

The reason organisms can be different from each other even though their genetic material is made of the same molecules is because the order of nucleotides is different.

The sequence of nucleotides is what forms the genetic information of an organism.

Base Pairing ReviewA T G C G T A T C

__ __ __ __ __ __ __ __ __

G C T G A G C G T

__ __ __ __ __ __ __ __ __

T A G C T C G G A

__ __ __ __ __ __ __ __ __

C G A C G T C A G

__ __ __ __ __ __ __ __ __

Base-Pairing Review1. How can two organisms be so different

from each other if their genetic material is made of the same DNA --- G, C, A, T?

2. _____ and _____ are called purines because they are a _______ ring of carbon and nitrogen.

3. _____ and _____ are called pyrimidines because they are a _____ ring of carbon and nitrogen.

The Central DogmaThe central dogma describes how

information from DNA gets used to make proteins.

The Central Dogma involves three processes:1. Replication – copies DNA2. Transcription – converts a DNA message

into RNA 3. Translation – interprets RNA into a string

of amino acids, called a polypeptide

DNA REPLICATIONDuring mitosis and meiosis, the cells

divide. Each time a cell divides, it must make a copy of its DNA.

Replication is the process by which DNA is duplicated, forming two identical copies from one original.

3 Steps to DNA Replication1. The enzyme helicase breaks the hydrogen bonds

between the nitrogen bases that hold the two strands together – effectively unzipping the DNA molecule.

2. As the DNA continues to unzip, free nucleotides, from the surrounding area in the nucleus, bond to the two single strands according to the base pair rule.

3. The enzyme DNA polymerase forms the sugar-to-phosphate bonds that connect the nucleotides on each strand of DNA.

DNA REPLICATIONReplication of DNA doesn’t begin at

one end of the molecule and proceed to the other.

It occurs simultaneously at many points on the molecule, speeding up the process.

Replication is completed when the entire molecule has been unzipped and replicated.

DNA REPLICATION VIDEO

https://www.youtube.com/watch?v=8kK2zwjRV0M

TRANSCRIPTIONTranscription is the process of producing

RNA (Ribonucleic Acid) from DNA.

RNA is the form of a nucleic acid in which information moves from DNA in the nucleus to the ribosomes in the cytoplasm.

The process of transcription is similar to the process of replication, with one exception:Adenine pairs with Uracil – not thymine

3 DIFFERENCES BETWEEN DNA & RNA

1.RNA is single stranded – DNA is double stranded

2.RNA’s sugar is ribose – DNA’s sugar is deoxyribose

3.RNA has the base uracil – DNA has the base thymine

3 TYPES OF RNA1. mRNA – messenger RNA

It carries the information from DNA (in the nucleus) out into the cytoplasm.

2. tRNA – transfer RNA It brings amino acids to the ribosomes so they

can be assembled into proteins.

3. rRNA – ribosomal RNA It makes up the ribosomes, which are the site

of protein synthesis.

COMPARE & CONTRASTREPLICATION & TRANSCRIPTION

SimilaritiesBoth involve unwinding the DNA double helixBoth involve large enzymes called polymerases

DifferencesReplication makes an identical copy of DNA, but

transcription makes RNA moleculesReplication happens only once during the cell

cycle, but transcription happens over and over on the same gene to make many copies

Transcription Practice

1.GATACGATGACG

2.TACGTTACGCCA

3.GGTACAACGTGT

4.GATCGATCAAGC

5.GCAAACTTACGAG

6.CGATAGCATCGAT

TRANSLATIONTranslation is a process that converts a message

from one language into another, likewise, Translation happens in your cells too.

Cells translate an mRNA message into amino acids.

A long strand of amino acids are known as a polypeptide, which are the building blocks of proteins – this whole process is called translation.

An mRNA message can be translated into 20 different amino acids.

TRANSLATION, CONTINUED

The genetic code consists of four letters that make up 3-letter “words” called codons.

Each 3-letter codon codes for 1 of the 20 amino acids.

Multiple codons can code for the same amino acid. For example, the codons UCU, UCC, UCA, and UCG all code for serine.

TRANSLATION, CONTINUED

In addition to codons that code for amino acids, there is a start codon & three stop codons.

The start codon (AUG) signals the start of translation & also codes for the amino acid methionine.

The three stop codons (UAA, UAG, & UGA) signal the end of an amino acid chain.

TRANSLATION, CONTINUED

Why have a start codon anyway?For words to make sense, they must be read

starting at the correct place, or with the correct reading frame.

For example:T HEC ATA TET HER ATTH ECA TAT ETH ERA TTHE CAT ATE THE RAT

There are no spaces between codons, but the correct start site and the correct reading frame must be used for the message to make sense.

TRANSLATION, CONTINUED

The process of translation happens in the cytoplasm of both prokaryotic and eukaryotic cells.

The amino acid chain is bonded together with a peptide bond.

Transcription & Translation are key factors for protein synthesis.DNA RNA Amino Acids Protein

TRANSLATION PRACTICEAUGGUACUGUAA

GCUAUGUGAUCU

GUAUGCGUGGCGAGUAG

MUTATIONSA mutation is any mistake or change in the DNA

sequence.

Mutations in gametes can be passed on to offspring of the affected individual, but mutations in body cells affect only the individual in which they occur.

There are two types of mutations:1. Chromosome Mutations

2. Gene Mutations

CHROMOSOME MUTATIONA chromosome mutation involves a change in the

structure or number of chromosomes.

Explanation of chromosome mutations:

Nondisjuction is when one or more pairs of homologous chromosomes fail to separate during meiosis.

Deletion – a piece of chromosome breaks off completely. After cell division, the new cell will lack a certain set of genes. Often this is fatal to the zygote.

CHANGES IN CHROMOSOME STRUCTURE

Insertion (Duplication) – A chromosome fragment attaches to its homologous chromosome, which then carry two copies of a certain set of genes

Inversion – the chromosome piece reattaches to the original chromosome but in a reverse position

Translocation – the piece reattaches to a nonhomologous chromosome

GENE MUTATIONGene mutation involves a change in the chemical

makeup of the DNA.

Point mutations occur when one or more DNA nucleotides are deleted or substituted with others.

Frameshift are mutations that change one or just a few nucleotides in a gene on a chromosomeSickle cell anemiaCystic Fibrosis

MUTATIONSOccasionally random gene mutations produce

changes that make the individual better adapted to the environment. Such mutated genes tend to increase in frequency within a population

MUTAGENSMutagens are factors that increase the number of

occurrences of mutations.

Examples:Ultraviolet RaysRadioactive SubstancesChemicalsNicotineAlcohol