Microsoft Word - blank.doc

NEL Molecular Genetics 663

Section20.1

Learning Tip

The rules of complementary

DNA base pairing are

• A to T


• G to C
  When you know the sequence
  on one strand, you also know
  the sequence on the
  complementary strand.

antiparallelparallel but running in

opposite directions; the 5
end of

one strand of DNA aligns with the

3 
end of the other strand in a

double helix

Practice

1. Define the following terms: nucleotide, complementary base pairing, and antiparallel.
   2.In a DNA molecule, a purine pairs with a pyrimidine. If this is the case, then why
   can’t A–C and G–T pairs form? (Hint:Look closely at the bonds between the base
   pairs in Figure 2on the previous page.)
   3.The following is a segment taken from a strand of DNA: 5
   –ATGCCTTA–3
   . Write out
   the complementary strand for this segment. Be sure to show directionality.

DNA replicationthe process

whereby DNA makes exact copies

of itself

semiconservative replication

process of replication in which each

DNA molecule is composed of one

parent strand and one newly

synthesized strand

templatea single-stranded DNA

sequence that acts as the guiding

pattern for producing a

complementary DNA strand

miniInvestigation Building a DNA Model

What would a section of a DNA molecule look like if you could

see one close up? You can find out by building your own model

of the double helix. For this activity, you need to select

materials that will allow you to model the following features:

• the sugar–phosphate backbone


• the anti-parallel strands


• the four different nitrogenous bases
  
  
  • the bonds between complementary base pairs that hold the
    two strands together
    Your model should show a minimum of 12 base pairs. It should
    be free-standing and approximately 15 cm tall by 6 cm wide.
    Include a legend with your model that clearly identifies each
    part of the DNA strand.
  
  
  
  

DNA Replication

In Chapter 17, you saw that mitosis involves the duplication of chromosomes. For mitosis

to occur, DNA must copy itself and be equally divided between the daughter cells. To have

all the correct genetic information, the DNA in each daughter cell must be an exact copy

of the DNA in the parent cell.DNA replicationis the process by which a cell makes an

exact copy of its DNA. The main stages of DNA replication are the same in both prokary-

otic cells (without a membrane-bound nucleus) and eukaryotic cells (with a membrane-

bound nucleus).

DNA replication is semiconservative.Semiconservative replicationinvolves sepa-

rating the two parent strands and using them to synthesize two new strands (Figure 3,next

page). The hydrogen bonds between complementary bases break, allowing the DNA helix

to unzip. Each single DNA strand acts as a templateto build the complementary strand.

Finally, any errors are repaired, resulting in two identical DNA molecules, one for each

daughter cell.

The sequence of bases on any one strand of DNA can vary greatly between species, but

its opposite strand will always have the complementary sequence of bases. For example, the

sequences of the strands below are complementary:

5 
–ATGCCGTTA–3


3 
–TACGGCAAT–5

The two strands of nucleotides are antiparallel. They run parallel but in opposite

directions to one another. One strand will have a 5
carbon and phosphate group at one

end and a 3
carbon and the hydroxyl group of a deoxyribose sugar at its other end. Its

antiparallel strand will have a 3
carbon and the hydroxyl group of a deoxyribose sugar

at the first end and a 5
carbon and phosphate group at its other end (Figure 1,pre-

vious page).

The direction of the strand is important when enzymes interact with DNA, either to

copy the DNA prior to cell division or to “read” genes in order to make proteins. Enzymes

can read or copy DNA in only one direction. The sequence of only one DNA strand is

given when sequences are written out since the complementary strand is easily deduced

according to the rules of complementary base pairing.