Chromosome Structure & DNA Replication

When your cells are damaged, why are the new replacement cells structurally and functionally like the old damaged cells? Why can't we exchange cells (transplants)?

Chromosome structure

• DNA strands wind around histone proteins (nucleosome complex)
• Complexes are loosely coiled (chromatin) in nondividing cells and supercoiled & folded (chromosome) in dividing cells
• 23 pairs of chromosomes in each somatic cell, diploid

Watson-Crick Model of a DNA molecule

• Consists of two independent strands of nucleotides wound in a double helix
• Two strands are held together by complementary N-bases (H-bonds)
• Adenine complements Thymine, Guanine complements Cytosine
• ex. In class

   

   

   

   

   

   

DNA replication

Each new cell must have a copy of the parental DNA. How are the copies made?

• Copying process occurs in the nucleus
• Semiconservative process, bi-directional
• Requires supply of nucleotides, enzymes & DNA polymerase

Replication process:

1. DNA strands unwind (enzymes) & complementary nitrogen bases separate
2. DNA polymerase molecules attach to exposed nitrogen bases and promote binding of free nucleotides to complimentary nitrogen bases. Polymerase also links new nucleotides together
3. Each original strand serves as a template for a new complementary strand
4. New strand forms from 5’ end to 3’ end
5. New leading strand starts to form as parental strands unwind and forms as a continuous strand
6. New lagging strand starts to form at several points producing Okazaki fragments which are linked together by ligases
7. As DNA replication occurs histone proteins are added & new chromosomes formed