DNA (Deoxyribonucleic Acid)

DNA structure Codes for proteins Double Helix Double stranded Made of nucleotides 4 Bases Adenine Thymine Guanine Cytosine They base pair A-T, C-G

A closer look at DNA structure The strands run anti parallel to each other

the bases are pyrimidines and purines.

Hydrogen bonding in base pairing Weak hydrogen bonds

A nucleotide DNA has a 3’ and 5’ end (3 and 5 prime ends) based on the number of the carbons in the 5 carbon sugar. Nucleotides joining together to make DNA is a condensation reaction. Water is made

Condensing DNA into chromosomes Condensing DNA into chromosomes. DNA is not see as chromosomes unless cells are dividing. Normally the DNA is unwound and its instructions are being ‘read’. DNA is wound around histone proteins to form chromatin.

How does DNA replicate? DNA replication happens before a cell divides. In the S stage of the cell cycle. Note that one parent chromosome copies itself to form 2 strands called daughter strands which are held together by a centromere.

helicase and gyrase split and untangle the DNA strands ( not shown) DNA Polymerase can only work in a 5’ to 3’ direction. It adds free nucleotides and completes the leading strand in one piece. It copies the lagging strand in chunks called okazaki fragments The okazaki fragments are joined together by an enzyme called ligase

DNA is replicated in a semi conservative way with each original strand acting as a template. It happens all over the DNA at once in DNA puffs

The cell cycle G1 –first gap phase+ growth S- DNA Synthesis G2- second gap phase, checking and condensing Chromosomes

Cromatin

Karotype