INTRODUCTION TO DNA AND CHROMOSOMES

DNA: What is IT?

• DNA = Deoxyribonucleic Acid.• Inherited from your parents• Genetic material• Determines an organism’s traits

DNA Structure

• Double Helix --> twisted ladder • Made of nucleotides (building

blocks of DNA)• Nucleotides have three parts:

1. Deoxyribose sugar unit (Side)2. Phosphate group (Side)3. Nitrogenous base (Rungs)

4 Different Nitrogen Bases:

1. Adenine (A)2. Thymine (T)3. Cytosine (C)4. Guanine (G)

Always in pairs to form rungs of a ladder A with T & G with C

Ladders can UNZIP! Like a ZIPPER! (important in DNA Replication)

Importance of Nucleotide Sequences

• All organisms:• are composed of different proteins• have DNA made up of the same 4

nucleotides.• So… how can organisms be

different if their genetic material is made of the same 4 nucleotides?????

How Do Organisms Differ?

• By the way the nucleotides are sequenced in DNA

• Example: A-C-C-T-T-G vs. G-G-C-T-A-T. (Like the words STREAM and MASTER –

both have same letters, BUT different arrangement and meaning)

• If one tiny mistake occurs in one pair-- > a particular protein will not be made

Organisms with Similar DNA

• Organisms with very similar sequences are very closely related• Example: the chimpanzee and gorilla

Where is DNA Found?

Nucleus of cells Makes up the chromosomes

Chromosomes Long strands of DNA Each one contains many genes Homologous pairs

2 chromosomes that look similar & have the same genes

• Humans = 46 chromosomes (23 pairs)• 22 homologous pairs • 2 sex chromosomes (Male =XY, Female = XX)

Karyotyping:

• Picture of an individual’s chromosome

More on Karyotyping

• A cell sample is taken & chromosomes are photographed

• Homologous pairs are matched up• Normal Karyotype:

• 46 chromosomes (22 homologous, 2 sex chromosomes)

• Can detect chromosomal abnormalities in fetus by amniocentesis• Missing, extra, or broken chromosomes

Can you see the extra chromosome?....... This one shows an extra chromosome #21 – called trisomy 21. A person with this karyotype has Down’s Syndrome.

CHROMOSOMES versus GENES?

• Chromosome: made of many genes• Gene: DNA sequence that codes for a

trait• Allele: alternate form of a gene

• Examples: eye color gene has blue, brown, green alleles