DNADNADNADNA

Chapter 4 Section 3Chapter 4 Section 3

What is DNA?• Deoxyribonuclei

c acid• Hereditary material of a cell.• Chemical that stores

information for the growth and function of an organism.– Stores this info in the form of a

code.

• Copied and passed on to new cells each time a cell divides.

What is DNA?• Discovering DNA • Mid-1800’s

– Scientists knew that the nuclei of cells contained large molecules called nucleic acids.

• 1950– Scientists knew what the

nucleic acid DNA was made of but didn’t understand its structure

What is DNA?• DNA’s Structure • 1952

– Rosalind Franklin discovered DNA is two chains of molecules in a spiral form, similar to a twisted ladder.

• 1953– James Watson and

Francis Crick made a model of DNA.

What is DNA?• A DNA Model • Each side of the ladder is

made up of a sugar (deoxyribose) and a phosphate molecule.

What is DNA?• A DNA Model • Rungs of the ladder are made of 4

different nitrogen bases– Adenine (A), Guanine (G), Cytosine

(C), and Thymine (T)– Bases always occur in pairs

• Guanine will only pair with cytosine• Adenine will only pair with thymine• So,

– Amount of cytosine in the cell always equals the amount of guanine.

– The amount of adenine in the cell always equals the amount of thymine.

What is DNA?• Copying DNA 1. The two sides of the

DNA molecule unwind and separate.

2. The nucleotides on each side pair with free nucleotides to form two new DNA strands.

Genes• Gene • A section of DNA on a chromosome.

• Contains instructions for making a specific protein.– Proteins determine most of your

characteristics (by building tissues and cells and controlling chemical reactions (enzymes).

– Each protein is made from multiple amino acids.

• Genes control the order of the amino acids used to make the protein.

Genes• Making

Proteins• Genes are located in the

nucleus.• Proteins are made on

ribosomes in the cytoplasm.• So, codes for making the

proteins have to be carried from the nucleus to the ribosomes by RNA.

Genes• Ribonuclei

c Acid

• Made in the nucleus from the pattern of the DNA.

• Can be thought of as half of a DNA molecule.• Has one side made of a sugar (ribose) and a

phosphate group.• Contains 4 bases:

– Adenine, Guanine, Cytosine, and Uracil (U)

• 3 types– Messenger RNA (mRNA)– Ribosomal RNA (rRNA)– Transfer RNA (tRNA)

Genes• Making

Proteins1. mRNA is made in the nucleus

from the DNA pattern and than sent to the cytoplasm.

2. In the cytoplasm, a ribosome made from rRNA attaches to the mRNA.

3. tRNA brings amino acids to the ribosome.1. Each tRNA molecule will only bind

with a specific amino acid.

Genes• Making

Proteins4. Inside the ribosome, the three

nitrogen bases on the tRNA molecule bind with matching bases on the mRNA.

5. The ribosome slides down the mRNA chain to the next set of bases.

6. A new tRNA molecule carrying a second amino acid pairs with the new bases.

Genes• Making

Proteins7. The two amino acids that are now

adjacent to each other form a bond.8. The first tRNA releases its amino

acid and moves back to the cytoplasm to get a new amino acid.

9. Amino acids continue to be assembled in this manner until the protein is complete and released from the mRNA.

Genes• Controlling

Genes• Codes contained in the mRNA

determine the order that amino acids are assembled.– Different orders of amino acids

make different proteins.– If amino acids are assembled in

the wrong order, the protein won’t function properly.

Genes• Controlling

Genes• Each cell uses only some of the

thousands of genes that it has to make proteins.– Each cell only makes the proteins that

it needs.• So, cells have to control genes by turning

some genes on and some genes off.• Cells can turn genes on and off in many

ways, including:– Twisting so tightly that no RNA can be made.– Binding to chemicals so the DNA cannot be

used

Mutations• Mutation • Any permanent change in the DNA

sequence of a gene or a chromosome of a cell.– Occur when DNA is not copied properly.– Results in the production of the wrong

proteins, malfunctioning proteins, etc.

• Mutations can be caused by extra or missing chromosomes or by outside factors such as X rays, sunlight, or chemicals.

Mutations• Results

of a mutation

• Can:– be harmful (most) or beneficial– change the traits of the organism– be fatal– have no effect

• If in a body cell:– mutation only affects those cells made from the

body cell.

• If in a sex cell:– mutation is passed on to all cells in the organism

produced from the body cell.