chapter 4 dna & rna the nucleic acids remember: each chromosome is a very long dna molecule that...

Chapter 4

DNA & RNADNA & RNA The Nucleic AcidsThe Nucleic Acids

Remember: Each chromosome is a very long DNA molecule that contains many genes.

Gene: A segment of DNA that is part of a chromosome that is responsible for inherited traits such as eye color, hair color, skin color, etc.

It directs the protein production that controls the cell.

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DNA: (Deoxyribonucleic Acid):•Is an organic molecule composed of nucleotides.•Is a molecule that is common to all living things, from bacteria to humans.•It is the blueprint of an organism, containing the genetic instructions for building proteins.•A DNA molecule canNOT be viewed with a compound light microscope.

The composition of DNA was first described correctly in 1953 by two scientists, Watson and Crick. They discovered that it was formed from two long chains of nucleotides shaped much like a spiraling ladder, and they called it a double helix.

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Structure of DNA: (DNA is composed of nucleotides.)

Nucleotides of DNA have three Parts:1. Deoxyribose (a five carbon sugar)2. Phosphate group

(these two bond together to make up the sides of the ladder)

3. 1 of 4 Nitrogen BasesAdenine (A)Guanine (G)Cytosine (C)Thymine (T)

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The nitrogen bases adenine and guanine are called purines which have a double ring of carbon and nitrogen.

The nitrogen bases cytosine and thymine are called pyrmidines, which have a single ring of carbon and nitrogen.

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•The 2 chains of nucleotides in a DNA molecule are held together by hydrogen bonds between the nitrogen bases.

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•The 2 chains of nucleotides in a DNA molecule are held together by hydrogen bonds between the nitrogen bases.•The 2 bases on the same rung of the DNA ladder are referred to as a base pair.•In DNA

•Cytosine always bonds with guanine, and•Thymine always bonds with adenine!

C=G G=CT=A A=T

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•Base Pairing Rules. The strictness of base-pairing results in 2 strands that are complementary, which means the sequence of bases on one strand determines the sequence of bases on the other strand.

Example:1st strand of DNA has T C G A A T TThe other strand must have A C T A

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•All organisms contain the same DNA (made up of nucleotides with adenine, thymine, guanine, and cytosine)

•The reason organisms can be different from each other is because the order of nucleotides in two different organisms are different (sequence of bases)

Example: A squirrel differs from a rosebush because the order of nucleotides in its DNA is different.

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•Replication:During 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.

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These are the steps of replication:1. The enzyme DNA helicase breaks the hydrogen

bonds between the nitrogen bases that hold the two strands together, unzipping the DNA molecule.

2. As the DNA continues to unzip, free nucleotides from the surroundings in the nucleus bond to the single strands base pairing.

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

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Replication of DNA doesn’t begin at one end of the molecule and end at the other, rather it occurs simultaneously at many points on the molecule, speeding up to process.

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

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Each new DNA molecule has one nucleotide strand from the original DNA molecule and one nucleotide strand that has been newly synthesized from free nucleotides in the cell.

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Gene Expression is the use of genetic information in DNA to make proteins.

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Gene Expression takes place in 2 stages:

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Gene Expression takes place in 2 stages:1. Transcription is when the RNA copy of a gene is

made.

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Gene Expression takes place in 2 stages:• Transcription is when the RNA copy of a gene is

made.• Translation is the 2nd stage of gene expression

where 3 different kinds of RNA work together to assemble amino acids into a protein molecule.

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Gene Expression takes place in 2 stages:1. Transcription is when the RNA copy of a gene is

made.2. Translation is the 2nd stage of gene expression

where 3 different kinds of RNA work together to assemble amino acids into a protein molecule.

Those 3 different kinds of RNA that work together are mRNA, tRNA, rRNA.

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Transcription Transcription is the process of producing RNA

(Ribonucleic Acid) from DNA.

RNA is the form in which information moves from DNA in the nucleus to the ribosomes in the cytoplasm.

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Transcription The process of transcription is similar to the process of

replication, but RNA differs from DNA in 3 ways:

1. RNA is single stranded (DNA is double stranded)2. RNA has ribose as the sugar (DNA has deoxyribose

as the sugar 3. RNA has four nitrogen bases, but Thymine is

replaced with Uracil

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There are 3 Types of RNA:1. 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, the site of protein synthesis

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DNA & RNADNA & RNA Cytosine Cytosine

Guanine Guanine

Adenine Adenine

Uracil Thymine

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The Genetic CodeRemember that DNA’s purpose is to provide a blueprint

for making proteins.

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for making proteins. Proteins are built from chains of smaller molecules called amino acids, and there are 20 of them.

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for making proteins. Proteins are built from chains of smaller molecules called amino acids, and there are 20 of them.

Recall that there are only four nitrogen bases. So how can four bases code for 20 amino acids?

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The Genetic CodeThe answer is that each strand of DNA is read in sets of

three nitrogen bases, called a codon.

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The Genetic CodeThe answer is that each strand of DNA is read in sets of

three nitrogen bases, called a codon. By reading in groups of three bases there are 64 combinations possible.

Of these 61 code for amino acids, and there is a START codon (AUG) and 2 STOP codons (UAA & UAG)

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Protein Synthesis Equation:

DNA mRNA Protein

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Protein Synthesis Equation:

DNA mRNA Protein transcription translation

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TranslationTranslation (or protein synthesis, gene expression) is

the process of converting the information in a sequence of nitrogen bases in mRNA into a sequence of amino acids that make up a protein.

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TranslationThe steps of translation are:

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TranslationThe steps of translation are:1. The first codon of the mRNA strand attaches to a

ribosome (rRNA).

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TranslationThe steps of translation are: 1. The first codon of the mRNA strand attaches to a

ribosome (rRNA).2. Then a tRNA molecule has a special section called

an anti codon that is complementary to the codon on the mRNA molecule. The first codon is methionine. AUG signals the start of protein synthesis.

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3. Next the ribosome slides down the mRNA strand to the next codon.

4. When the first and second amino acids are in place, an enzyme joins them by a peptide bond.

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5. This process continues until it reaches a stop codon. The chain of amino acids are now considered a protein.

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The order of events that leads to genetic expression:Protein synthesis

• DNA to RNA to amino acids to protein

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Mutations:

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.

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Mutations:1. Chromosome mutation –involves a change in the

structure or number of chromosomes.One example is nondisjunction (when one or more

pairs of homologous chromosomes fail to separate during meiosis.)

Down Syndrome in humans is caused by the presence of an extra 21st chromosome.

Other chromosome mutations may be caused by deletion, translocation, inversion, and duplication of parts of the whole chromosome.

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Four Types of Mutations:

1. Deletion mutation- a piece of chromosome breaks off completely.

The new cell will lack a certain set of genes. Often this is fatal to the zygote.

2. Duplication mutation- a chromosome fragment attaches to its homologous chromosome, which then carry two copies of a certain set of genes.

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Deletion Mutation Duplication Mutation

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3. Inversion mutation- the chromosome piece reattaches to the original chromosome but in a reverse position.

4. If the piece reattaches to a nonhomologous chromosome a translocation mutation results.

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Translocation Mutation

Inverse Mutation

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Mutations:• Gene mutation—involves a change in the chemical

makeup of the DNA. This is where one or more DNA nucleotides are deleted or substituted with others. These are called point mutations.

Frame shift—are mutations that change one or just a few nucleotides in a gene on a chromosome.

Example: sickle cell anemia and cystic fibrosis

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Occasionally random gene mutations produce changes that make the individual better adapted to the environment. Such mutated genes tend to increase in frequency with in a population.

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Mutations:3. Mutagens increase the incidence of mutation.

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Mutations:1. Mutagen mutation increase the incidence of

mutation.Examples of these:

XraysUltraviolet raysRadioactive substancesChemicalsNicotineAlcohol

chapter 4 dna & rna the nucleic acids remember: each chromosome is a very long dna molecule that...

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