DNA AND ITS STRUCTURE

DNA is located inside the nucleus of eukaryotic cells

DNA is stored in chromosomes; a gene is a segment of DNA which codes for proteins

A karyotype is a picture of all the chromosomes in a cell. Humans have 22 pairs of chromosomes and

two sex chromosomes. This is an abnormal karyotype. Can you find the mistake?

Female and Male Sex Chromosomes. Everyone needs an X chromosome as it contains critical genes that make us human.

DNA

DNA

Heredity material for life

Deoxyribonucleic Acid

Double helix Molecule

What has DNA?

--Bacteria

-People-Animals-Plants- yes, this means the fruit and vegetables you eat contain DNA-Basically, if it is alive, or once was alive, it has DNA

DNA

• Your DNA contains a set of instructions for building a human. It is responsible for all our inherited characteristics and is passed down to us from our parents

• It directs all the cell’s activities

• Instructions in DNA codes for proteins (proteins are responsible for thousands of chemical reactions that take place in our bodies and also codes for our traits)

So, DNA is like a recipe or blue print for making us human

A dog’s DNA has instructions different than ours-it “builds” a dog

Did you know that your DNA is 99.9% like everyone else’s in this room? So, just a tiny bit of difference in

DNA can make us look very different.

But your DNA is MORE than 99.9% like your relative’s DNA

Did you know that 50% of the genes in a banana are the same as your genes? That’s right! You have lots in common with this banana which is probably why he is

smiling and waving at you.

The DNA in a flower codes for their special traits….making them unique to other flowers

• DNA shows us how living organisms can pass down information to their offspring

• For example, it tells us how a child can be born with “his mother’s eyes” or “his father’s nose”

• DNA is compact, yet complex enough to carry all the information needed to guide the development of an entire organism

Just as we get our DNA from our parents, plants inherit their DNA from the plants they came from.

DNA USES• Help to solve crimes (find the criminal)

• Helps to identify victims of a crime or accident

• Has been used to identify genetic illnesses

• Used to determine blood/biological relatives (such as in paternity tests)

• Used in cloning, gene therapy and genetic engineering

Where Can You Get A Sample of DNA From People?

• Skin cells

• White blood cells (white blood cells only, not red blood cells because they do not contain a nucleus)

• Bones

• Semen

• Saliva

• Hair

Do Identical Twins Have the Same Fingerprints?

• The same DNA- yes!• The same fingerprints? No!• True, fingerprints are a phenotype (physical

characteristic) that are determined by the interaction of genes, but also the development of fetus in uterus has an influence here

• The shape of fingerprints is mostly influenced by environmental factors during pregnancy, like nutrition, blood pressure, position in the womb, and the growth rate of fingers in the first trimester. SO TWINS WILL HAVE similarities in fingerprints, but differences, too!

Exciting DNA!

• What makes DNA so exciting is that it shows how living organisms store information in biological molecules

• The structure of DNA is nicely suited for this task• The structural backbone creates a simple,

consistent chain upon which many, many bases can be laid out in an orderly sequence

Double Helix Molecule-DNA

James Watson and Francis Crick

In 1953, these two scientists made history by discovering the molecular structure of DNA.

What is this structure? DNA is a double helix

nucleotide made up of four bases, a phosphate, and the sugar, deoxyribose

The Nitrogen (nucleotides)Bases

• These bases make up the “rungs” of the ladder:

Adenine (A) Thymine (T)

Guanine (G) Cytosine (C)

DNA is made up of a long sequence of

these units strung together.

So the genetic code is written in a series of A’s, T’s, G’s, and C’s. It’s a “secret” code that scientists are trying to crack- to figure out just how it is that DNA

makes us all unique .

• The capital letters represent the bases:

Adenine= A

Thymine= T

Guanine= G

Cytosine= C

Pairing of the Nucleotide Bases

Adenine always pairs with thymine

Guanine always pairs with cytosineEach base has a particular shape that causes this base pairing

The back bone of DNA

• Made up of alternating molecules of phosphate and a sugar called deoxyribose

• Base pairs are complementary to each other because each base only bonds with the other base pair

• CCGA will bond to the sequence __________

• GGCT

• AATTCGAT will have a complementary strand of:

• AATTCGAT

TTAAGCTA

Before a cell divides, the DNA in every chromosome is copied.

• This is called DNA replication

How Does DNA Replicate?

First, in replication, a DNA molecule is split down the middle by the enzyme, DNA helicase

The bases on each side of the molecule are used as a pattern for a new strand

• As bases on the original molecule are exposed, complementary nucleotide bases (from the nucleus) are added. DNA polymerase brings them together then proofreads it for mistakes

• DNA ligase is an enzyme that seals in the sugar and phosphate backbone.

So half of the new molecule is old DNA and half is new DNA

What are Mutations?• It is a change in the nucleotide- base

sequence of DNA, in other words: a mistake with the pairing of the A,T,C, and G’s

• It can be caused by random errors in the copying of DNA

• Or by things in the environment

Do Mutations Matter?

• Changes in DNA can cause an improved trait, no change, or a harmful trait

• If the mutation does not change the protein that the gene makes then there will be no change to the trait

Can you think of a mutation that could benefit an organism?

When a nucleotide is deleted or inserted, the result is a polypeptide that codes for the wrong sequence

of amino acids• Consider this for ex: THE CAT ATE THE RAT. If the letter C is deleted

from this sentence, then it would read: THE ATA TET HER AT- something that doesn’t

make sense! Mutations can lead to proteins that function

poorly or not at all- this may or may not be of concern, it depends on what protein it is coding for. If there is a mutation in the hemoglobin of red blood cells that affects its shape, this could cause sickle cells that lead to blood clots (sickle cell anemia).

Errors in DNA replication are rare, because DNA polymerase proof reads the new and old strand,

but still they sometime do occur

Where the Mutation Occurs ..Mutations is a person’s skin cell won’t get passed down to an offspring, but mutations in a sperm or egg (gamete) can get passed down to one’s offspring and cause a genetic condition.

Known Harmful Mutagens (a mutagen is something from the environment that can

cause a mutation in DNA)

Some mutations in DNA lead to

• Diseases such as cancer (UV light can case changes in skin cell’s DNA)

• Physical signs in aging

A mutagen might have caused this frog to develop six legs

Note Check

Is a mutation always harmful? No they are not always harmful- sometimes they

are beneficial, other times they cause no change in the organism

*Does a mutation always produce a change in the individual? Explain.

No, only if it changes a protein that a gene codes for will there be a change in a trait

• But there is so much information that needs to be stored about us. How is DNA able to provide enough information for a living organism?

• The simple answer is that DNA molecules are very, very long

• Did you know a simple bacteria, E. Coli, is 4 million nucleotides sequences long! This corresponds to the information storage capacity of an 8 MB hard drive! Lots of information for a tiny bacteria!

The human genome is approximately 33billion nucleotides long! billion nucleotides long!

The genome is all the DNA in our cell.

The information

storage capacity of a 3GB hard drive on your computer!

We can think of DNA as a “genetic

database” for living organisms . We have somewhere between 25,000 and 30,000 genes.

Genes can be isolated and studied, and used to try and treat diseases/conditions.

What is the human genome project?• A project designed by the U.S. department of

Energy and National Institute of Health to:• Their goal is to identify all the approximately

30,000 genes in human DNA• To determine the sequence of the 3 billion

chemical base pairs (nucleotides)• Store the information in databases• improve tools for data analysis• Transfer data to private sector• Address ethical issues stemming from the

project’s findings

In other words, scientists want to locate every gene in our genome (all our DNA). Then this information

will be mapped out and stored in a database or book- kind of like an encyclopedia !

A cloning cartoon

DNA Extracted From Strawberries

Uses of Protein in the Body• Proteins play a role in the anatomy and physiology of

cells• The plasma membrane contains protein that carry out

various functions• Many proteins are enzymes that are responsible for

thousands of chemical reactions in our bodies• Some proteins are hormones that regulate the body’s

functions• Antibodies in blood and hemoglobin are proteins…..• Genotypes and phenotypes are determined by proteins

Before we talk about protein synthesis let’s talk about another important nucleic acid in the body..RNA

• DNA• Double stranded

molecule• Contains the sugar

deoxyribose• Contains the base

pairs: A,T,G,C

• RNA• Single stranded

molecule• Contains the sugar

ribose• Contains the base

pairs: A,U,G,C

Types of RNA

• There are three types of RNA:

1.Messenger RNA (mRNA)

2. Ribosomal RNA

(rRNA)

3.Transfer RNA

(tRNA)

Protein Synthesis• Protein synthesis takes place in the ribosomes• A gene is a segment of DNA that specifies the

amino acid sequence of a protein• DNA stores this information in a nucleotide code• A difference in base sequences can determine

whether you have blue eyes or long fingers and more!

• A gene does not directly control protein synthesis, however, it passes its genetic information to RNA which is more directly involved.

Gene expression requires two steps

Transcription

Genetic information is passed from DNA to RNA

One type of polynucleotide (DNA) is transcribed letter by letter into another type of polynucleotide (RNA)

This takes place in the

nucleus

Translation

Here the RNA transcript directs the sequence of the amino acids in the newly formed protein

This step takes place in the cytoplasm and in the ribosomes

Step 1 Transcription

• Transcription begins when the enzyme RNA polymerase binds tightly to a promoter, a region of DNA that contains a special sequence of nucleotides.

• This enzyme opens up the DNA helix so that complementary base pairing can occur

• Then RNA polymerase joins the RNA nucleotides and an mRNA molecule results

The nucleotide code comes in a triplet code, in order to supply 64 different triplets, far more than

needed to code for the 20 known amino acids!

A codon is a group of three base pairs that code for a specific amino acid on an mRNA molecule…

What two amino acids are coded for here?

Anticodons

• Anticodons are a group of three bases that are complementary to a specific codon of mRNA

• There is at least one tRNA for each of the 20 amino acids found in proteins

• tRNA bring amino acids to the ribosomes and pair them up accordingly with the codon on mRNA

Use chart on P.492 worksheet

Codon (mRNA) Anticodon (tRNA) Amino Acid

CAA GUU glutamine

Translation takes place in the cytoplasm and new proteins are

made in the ribosomes

Translation requires 3 Steps

• Chain initiation

• Chain elogation

• Chain termination

A small ribosomal subunit binds to mRNA; an initiator tRNA with the anticodon UAC pairs with the codon AUG

A tRNA amino acid approaches the second binding site of the ribosome. Peptide bond formation attaches the polypeptide chain to the newly arrived amino acid

The ribosome comes to a stop codon on the mRNA. A release factor binds to the site. The release factor

hydrolyzes the bond between the last tRNA at the Psite and the polypeptide, releasing them. The ribosomal units

dissociate.

Transcription and Translation

What are some practical benefits to learning about DNA?

• Knowledge of the variations of DNA among individuals can help diagnose, treat and someday prevent thousands of disorders that affect us

• Useful in crime solving cases and paternity suits

• Help us understand the natural capabilities of our non- human friends- that can be applied in solving challenges in agriculture, health care, energy production, etc

Questions for guess who?

• That’s right…for you!

Questions

1.Which two scientists are well known for

discovering the structure of the DNA

molecule?

Francis Crick and James Watson

3.The nitrogen base pairing pattern is key

to understanding how______occurs.

DNA replication

• DNA replication ensures that each daughter cell will have____________.

All of the genetic information it needs to carry out its activities

4.DNA replication begins when DNA___________unzips the two sides of the DNA molecule causing it to unwind and separate, like a zipper unzipping in a jacket.

6.The molecule separates between

the__________on each rung.

paired nitrogen bases

Place the steps of DNA replication in order

1. The nitrogen bases that are floating in the nucleus pair up with the bases on each half of the DNA molecule

2.The two sides of the ladder unwind and separate, like a zipper unzipping

3.The new bases are attached 4.The order of the bases in each new DNA

molecule will match the order of the old DNA

• 2, 1, 3, 4

Warm-up

If this was a sequence of nitrogen bases in a DNA strand, what would its matching strand look like?

A T G C G A T C A A C

T A C G C T A G T T G

Warm Up

1. List three facts about DNA (book pages 57-59).

• DNA is shaped like a twisted ladder. This shape is known as a double helix.

• The two sides of the DNA ladder are made up of molecules of sugar, called deoxyribose, alternating with molecules called phosphates

• The rungs of the ladder are made of nitrogen bases