DNAA Look at Its Structure & Function

DNA• Is often called a “double helix”

–Twisted ladder

• That means that it has two strands that twist around each other. (double helix)

• The strands are made up of four nitrogen

bases:– Adenine– Thymine– Cytosine – Guanine

• The two sides of the ladder are made up of molecules of sugar. (deoxyribose) alternating with molecules known as phosphate.

C5H10O4

PO4

• The two strands of DNA complement each other.

• The nitrogen bases pair up in a certain way.

• Adenine always pairs with Thymine

• Guanine always pairs with Cytosine

Complementary Strands

• There are only four letters that make up the

language of DNA.– A, T, C & G (Adenine, Thymine, Cytosine &

Guanine)• Wherever there is an A you will have a T, Where

there is C you will have a G and so on…. Called

CHARGOFF’s RULE (or the complementary base pairing rule

Hydrogen Bonds-between a pair of bases-a WEAK bond

3 hydrogen bonds between guanine and cytosine

2 hydrogen bonds between adenine and thymine

Figure out what the complementary strand (other side) would be to these bases:

• ACG TAC GAA TCG ACG

• You should have:– TGC ATG CTT AGC TGC as a

complementary copy of the strand on the previous slide.

• Each group of three bases listed above, for example TGC is considered a codon.

• A codon is a group of three nitrogenous bases that codes for a particular amino acid.

Code Letters: Most of the time Scientists do not want to write out the names of the bases so they refer to them by their first letters.

•Adenine: A

•Thymine: T

•Cytosine: C

•Guanine: G

Nucleotides• DNA is made up of puzzle

pieces called nucleotides.

• Each nucleotide is composed of:– One deoxyribose sugar– One phosphate group– One nitrogenous base (A,

T, C or G)

A Nucleotide

How do we know all this???

•

James Watson & Francis Crick

• Crick and Watson, together with Maurice Wilkins, won the 1962 Nobel Prize in Medicine for their discovery of the structure of DNA. This was one of the most significant scientific discoveries of the 20th century.

Where is DNA Found?

• DNA is found in the Nucleus of the cell.

• DNA makes up the chromosomes in the nucleus of the cell.

• DNA duplicates during the S phase of interphase so that each new cell will have a complete copy of DNA.

DNA Replication

1. Replication means: making a copy of DNA.

2. It occurs during the S phase of interphase.3. There are four important components for

DNA replication to take place:1. Helicase2. DNA polymerase3. Ligase4. Free nucleotides

1. Helicase: An enzyme that attaches to DNA and unwinds the strand by breaking the hydrogen bonds between the bases .

• Ligase: is an enzyme that can connect two strands of DNA together. It works like glue to connect the parent strand to the daughter strand.

• DNA polymerase is an enzyme that assists in DNA replication binding to a single strand of DNA and initiating the attachment of nucleotides to the DNA strand.

Free nucleotides start to fill in the open sides of the nitrogen bases. Complementary base pairing must happen. (Adenine with Thymine, Cytosine with Guanine)

• When all bases are matched up you will have two identical strands of DNA.

• The cell will now enter the G2 phase of cell division preparing to begin Mitosis

FYI

A complete strand of human DNA is approx. 2 meters in length, and is composed of a chain of about 2.8 billion nucleotides .

The End