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2Importance of DNA

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• By the beginning of the 1900’s, it was known that the chromosomes carry the genetic or hereditary information.

• Heredity is controlled mostly by genes and to some degree by the environment.

• For example, all plants can make chlorophyll (which makes them green). There is a gene that gives them this ability. If a seed is grown in the dark, the leaves will come up white, not green (environment played a role.)

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• The hereditary material is DNA (deoxyribonucleic acid).– DNA makes up genes

– Genes make up chromosomes

•Another example: Identical twins have identical genes but may have some differences if raised in different places or eat different foods etc.

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• DNA is a nucleotide made of 3 parts:

P

S Base

1. A phosphate group

2. A deoxyribose molecule, which is a sugar molecule with 5 carbon atoms.

3. A nitrogenous base, which may be one of four kinds:

AdenineThymineGuanineCytosine

nucleotide

6DNA molecules

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Watson-Crick Model of DNA• James Watson and Francis Crick

developed a model of DNA.• According to this model, the

DNA consists of two chains of nucleotides in a ladder-like structure.

• The sides of the ladder are made of alternating phosphate and deoxyribose (sugar).

• A nitrogenous base is attached to each sugar molecule.

• The steps of the ladder are held together by weak hydrogen bonds between the bases.

8Watson & Crick 2 mins.

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• Only two base pair combinations can form in the DNA molecule

– Adenine (A) with Thymine (T)

– Guanine (G) with Cytosine (C)

– So A-T, T-A, C-G, and G-C are the only possible base pair rungs of the DNA ladder.

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Question…

• If one strand of the DNA molecule has the base sequence below, draw the other strand.– A - – T - – G -– C -

C

A

T

G

13Parts of DNA 4 min.

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• The DNA ladder is twisted to form a structure called a double helix (like a spiral staircase).

• Watson and Crick discovered the structure of the double helix. They also discovered how DNA replicates (copies itself) during mitosis and meiosis.

DOUBLE HELIX

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Review of DNA 1 min.

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Watson and Crick found that DNA:

• STEP 1: DNA unzips itself at the base pairs along the weak hydrogen bonds.

•STEP 3: The new double stranded molecules that form are identical to the original molecule.

•STEP 2: Free nucleotides that are present in the nucleus attach themselves by new hydrogen bonds to the free bases.

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21Replication of DNA 1.15

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G

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G

A

C

C

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A

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Gene Control - The Genetic Code

• A gene is a particular section of a DNA molecule.

• It is believed that the base-pair sequence (order) of the gene controls cellular activity by specifying a protein.

• The nitrogenous bases are like letters in an alphabet. The DNA molecule is a sequence of bases that spell out words in the DNA code. The code is eventually converted into an amino acid sequence to make the appropriate proteins needed by the organism.

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• The code is a triplet code.

• Every word has 3 letters in it.

• Each 3 letter sequence of bases on DNA template will lead to a complementary 3 letter sequence called a codon.– Ex: CAG, GGC, ACG

• Every codon (3 bases) will code for one amino acid that will be put in place to make a protein.

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• DNA (with its important code) stays inside the nucleus.

– Proteins are made outside the nucleus on organelles called ribosomes.

• The DNA needs help to get the information outside the nucleus to make the proteins it codes for.

• One strand of DNA will act as the template for a different nucleic acid that can leave the nucleus.

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• RNA = ribonucleic acid.• This form of nucleic acid helps DNA

perform the synthesis of proteins. • RNA is different from DNA because:– It is single stranded.– It has ribose as its sugar.– The base thymine (T) is replaced by

uracil (U).

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The drawing is RNA because it has U as a base And not T.

You can also tell that it is RNA because it is single- stranded.

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There are 3 types of RNA…• 1. Messenger RNA (mRNA)

– This is the RNA copy of the DNA

• 2. Transfer RNA (tRNA)– This transfers the amino acids onto

the mRNA.

• 3. Ribosomal RNA (rRNA)– This is where the construction of the

protein takes place.

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• Each group of 3 nucleotides (bases) is called a codon.– Examples: AUU, GCC, UCG

• Each amino acid is coded for by a particular codon (3 nucleotides).

• Each transfer RNA carries an anticodon of three bases that matches the codon.

• This ensures that the right amino acids go into the protein.

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QUESTION:• If the DNA code is ACT AGG

CCA…..

The mRNA will be:

The tRNA will be:

UGA, UCC, GGU (codons)

ACU, AGG, CCA (anticodons)

The transfer RNA will bring the correct 3 Amino acids to the ribosome.

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•Transcription–Production of mRNA from DNA to leave nucleus with code.

•Translation–The production of proteins (amino acid chain) from a nucleotide chain.

44Protein Synthesis 3 min.

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ModernDNA

Technology

1. Cloning

2. Genetic Engineering

3. Gene Therapy4. DNA Fingerprinting

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– Identical copies of an organism without fertilization.

– Egg cell remove monoploid nucleus add diploid nucleus from a mammary cell or any other cell of an adult organism egg cell with new nucleus is transplanted into uterus of another organism new individual produced is identical to original.

– Example: Dolly (sheep) cloned in 1997 by Dr. Ian Wilmut

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• 2. Genetic Engineering– RECOMBINANT DNA = DNA combined

from 2 different organisms to produce characteristics not found in nature.

– Uses restriction enzymes to split DNA.– The sticky ends at the cut can be

touched to another DNA piece and adhere to it.

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Example of Recombinant DNA:• E. Coli

– E. Coli is a bacterium with a large DNA molecule and 2 smaller DNA rings called plasmids.

– A plasmid was spliced and the gene that codes for insulin was transplanted into bacteria.

– Bacteria reproduce often so it made a lot of insulin very fast.

– This is also being done with growth hormone and other needed substances.

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58Genetic Engineering 4 mins.

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Molecule A was made using Recombinant DNA.

A human gene for insulin was Combined with the DNA of a

Bacterial cell.

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-Gene therapy = a technique in which scientists hope to add “normal” copies of genes into cells of people who do not inherit the normal genes for given substances and end up with diseases.

3. Gene Therapy

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• 4. DNA Fingerprinting– Extract a person’s DNA– Use restriction enzymes to cut them into

fragments– Pieces will end up in different sizes from

different people.– DNA fragments are separated by electrophoresis.

Lines formed by each person’s individual DNA will be unique.

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• A gene mutation is a change in the chemical structure of the genetic material, DNA.

• Most gene mutations are recessive, like albinism (lack of skin pigments).

• Mutations are usually disadvantageous (not good) to the individuals in their normal environment and some may be lethal (can kill).

• Mutations on the other hand, are the basis for evolution. Sometimes mutations are good!

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Agents that cause mutations…• Any substance that causes a mutation is called

a mutagenic agent.• Examples of mutagenic agents….

– X-rays– ultraviolet rays– cosmic rays– radiation– formaldehyde– benzene– asbestos

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74The Genetic Code

75Genetic Counseling 9 minutes