DNA Structure/Replication & Chromosomes Notes

 

I.                 Nucleic acids

A.     Nucleic acids are a group of macromolecules that carry the genetic code for life.

B.     Examples are

1.      DNA- Deoxyribonucleic acid (De- without, oxy-oxygen)

2.      RNA- Ribonucleic acid

 

II.                DNA structure

 

A.     The subunits of DNA are called Nucleotides.

-Composed of three main parts:

1.      Phosphate group

2.      A 5-carbon sugar called deoxyribose

3.      A nitrogen containing base /There are four different types:

a.       Adenine   “A”

b.      Thymine  “T”     

c.       Guanine   “G”      

d.      Cytosine   “C”

 

B.      DNA is a Polymer of Nucleotides:

a.       In DNA, two strands of nucleotides are bonded and twisted around each other to form a double helix or twisting ladder. (Watson & Crick model is called the Double Helix)

b.       The sides of the DNA ladder are made up of alternating sugar and phosphate units.

c.       Attached to each sugar is one of four different nitrogen-containing bases: (A, T, G, or C)

 

d.       The two strands of nucleotides are held together by weak hydrogen bonds between the nitrogen bases.  They bond in the following way: (creating base pairs)

1.       “A” bonds to “T” and “T” bonds to “A”

2.       “G” bonds to “C” and “C” bonds to “G”

 

III.             DNA Replication -How DNA Copies Itself

 

A.      Many cells in our bodies will divide for growth, repair, and maintenance.  When one does, this original cell must first make an exact copy of it’s DNA to give to the newly forming cell.  This process of copying DNA is called DNA replication.  Usually, replication of DNA does not happen unless the cell has committed to dividing into two. (This happens in the “S” or Synthesis phase of the cell cycle.  We will discuss this later.)

B.     How does DNA make a copy of itself?

1.      The weak hydrogen bonds between the nitrogen bases in the DNA break apart the ladder forming two half ladders.

2.      The reason DNA “unzips” is because each half ladder will become a template to form a new DNA strand. (A template is an original in which copies can be made.)

3.      Then, DNA polymerase (an enzyme) attaches to the DNA and begins to attach new complimentary bases to the exposed half ladder.  New hydrogen bonds form creating the following base pairs:

a.       A-T and T-A

b.       G-C and C-G

4.      Sugar and phosphate groups are then covalently bonded to the new bases and to each other completing the other half of the DNA ladder.

5.      Proofreader enzymes go back and repair any mistakes made during replication. 

 

 

IV.           Chromosomes and DNA

1. Chromosomes are made up of coiled DNA and proteins called histones.  The histone acts like a spacer, preventing the DNA from kinking as it coils and uncoils.  Together one DNA and histone unit is called a nucleosome.   A stringy DNA strand with it’s nucleosomes is called chromatin.  A coiled chromatin would make a chromatid.  Two identical sister chromatids would make a chromosome.

There are 46 chromosomes in every human body cell.