All About DNA

All about DNA

• Deoxyribonucleic acid• A molecule that carries genetic information

from parent to offspring• It is the molecule that makes up the

chromosomes• DNA is a nucleic acid---one of the organic

compounds we already studied

DNA---brought to you by…….

• Rosalind Franklin: in 1952 she used XRAYS to first capture photos of DNA

• Watson and Crick: in 1953, they built on the information Franklin gave them and made a model of DNA that looks like a twisted ladder or a DOUBLE HELIX

The Structure of DNA

• DNA looks like a double helix (twisted ladder)

• It is made up of two strands that bond together

• It is called a double stranded molecule

The Structure of DNA

• The sides of the ladder are made up of:– SUGAR (the sugar

is called deoxyribose)–PHOSPHATE

deoxyribose

phosphate

Structure of DNA

• The rungs of the ladder are made up of NITROGEN BASES– There are 4 kinds of

nitrogen bases: adenine, guanine, cytosine, thymine (A, T, G, C)

– each rung has 2 nitrogen bases that pair up

– A & T, C&G always pair up

DNA Replication

• During interphase before mitosis and meiosis, the DNA is duplicated or copied or REPLICATED

• The copying of the chromosomes or copying of the DNA is called replication

• One double stranded DNA molecule will make two double stranded DNA molecules

Steps to DNA Replication

1. The enzyme (a special protein) called helicase breaks the bond between the nitrogen bases and unzips the DNA ladder

2. In the nucleus, there are free floating nitrogen bases

3. Another enzyme called DNA polymerase helps pair the free floating bases with the exposed DNA bases on the unzipped DNA

Steps to DNA Replication

4. New sugar and phospate groups form the sides of each new DNA strand5. Two new DNA molecules are formed (each one has 1 strand of the original DNA)

http://www.lpscience.fatcow.com/jwanamaker/animations/DNA%20Replication%20-%20long%20.html

Why do we care about DNA?• All of the characteristics or traits that you have

are due to the DNA that you have in your cells• Your eye color, hair color, ability to digest milk,

etc—these are your TRAITS• What TRAITS you have depends on what

PROTEIN your cells make• And what proteins your cells make depends

on your DNA CODE

PROTEIN

• Protein is an organic compound • Protein is made up of amino acids• Amino acids link together in a certain pattern• If you change the order of the amino acids you

change the protein • For example: insulin is a protein made of 51

amino acids; titin—found in muscle is 34, 350 amino acids long!

So we need protein to give us our traits……..How do we make protein?

• Proteins are made during a process called: protein synthesis

• if you recall what we learned about organelles:– Proteins are made in the ribosomes (which are in the

cytoplasm)– Your DNA code tells your body what proteins to make– But DNA is in the nucleus so how does it tell the ribosome

what to do because DNA cannot leave the nucleus?– This is where RNA comes in…..

RNA

• Ribonucleic acid• A single stranded molecule• It has ribose as its sugar (instead of

deoxyribose)• It is made in the nucleus but it CAN leave the

nucleus (and head to the ribosome!)• It does not have Thymine—instead it has

Uracil (U)

RNA

Two types of RNA

Messenger RNA (m-RNA)• RNA that copies the coded

message from DNA in the nucleus and carries the message to the ribosome

Transfer RNA (t-RNA)• RNA in the cytoplasm that

carries and amino acid to the ribosome and adds it to the growing protein chain

How to Make a Protein (Protein Synthesis)

• Act I: In the Nucleus…– mRNA is made in the nucleus– RNA polymerase temporarily unzips the DNA and

allows it to be copied---this creates the mRNA single strand

– The mRNA takes the copied code out of the nucleus and goes to the ribosomes http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/transcription.swf

How to make a protein (Protein synthesis)

• Act II: Meanwhile, in the cytoplasm….– Amino acids are floating around the cytoplasm– tRNA picks up amino acids and brings them to the

mRNA in the ribosome (each amino acid matches with a certain code on the mRNA)

– The amino acids that are brought to the mRNA bond together and as you put a chain of amino acids together, you will get a protein

– http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/translation.swf

Some final definitions:

• Chromosome: condensed DNA contains many GENES

• Gene: – a specific section of DNA on the chromosome– A gene tells the cell what protein to make; – Each gene controls different traits

• Genetic Code: the order of nitrogen bases along a gene—this tells the cell what protein will be produced

Mutations

• Sometimes there is an accidental change in the code during the copying of the DNA

• This changes the protein that is made• This causes a mutation• A mutation is any permanent change in a gene

or a chromosome• Some mutations are life threatening, others

will not even be noticed

Mutations

• Mutations might mean that:– A gene was broken– Codes can be rearranged or switched– An organism will receive too many chromosomes – An organism will receive one too few

chromosomes

– Ex. Down’s syndrome—extra 21st chromosome– Albinism—absence of a gene—no pigment