Download Open File

Name ___________________________
Date
________ Block__________
DNA and Genetics
DNA - The Double Helix
Recall that the nucleus is a small spherical, dense body in a cell. It is
often called the "control center" because it controls all the activities of the
cell including cell reproduction, and heredity. Chromosomes are microscopic,
threadlike strands composed of the chemical DNA (short for
deoxyribonucleic acid). DNA controls the production of proteins within the
cell. Proteins are the building blocks of an organism. The proteins that will
be made for your body are determined by the sequence of DNA in the
nucleus.
Chromosomes are composed of genes, which is a segment of DNA that
codes for a particular protein, which in turn codes for a trait. Hence you
hear it commonly referred to as the gene for blue eyes. Meanwhile, DNA is
the chemical that genes and chromosomes are made of. DNA is a nucleic acid
found not only in the nucleus but also in the mitochondria and chloroplasts. It
is the DNA in the nucleus that actually controls the cell's workings.
After the components of DNA were figured out, a
race among scientists began to determine the structure
of DNA. Three scientists are given credit to the
structure. Rosalind Franklin, a female scientist was
actually the first to determine the structure however
James Watson and Francis Crick are credited to
establishing it. X-ray photographs were used to
describe the shape and dimensions of the molecule.
DNA is made up of alternating nucleotides. Each nucleotide consist of a
nitrogenous base, a phosphate molecule and sugar. The shape of DNA is a
double helix (double stranded), which looks like a twisted ladder. The sides
of the helix are made of alternating sugar and phosphate molecules. The
sugar is named deoxyribose.
The rungs of the ladder consist of 4 different are pairs of 4 types of
nitrogen bases - adenine, thymine, cytosine, and guanine. The bases are known
1
by their coded letters - A, T, C, and G. The strands of the molecule are
connected by complementary nucleotide pairs (A & T and C & G) like rungs on
a ladder. These bases always bond in a certain way. Adenine will only bond to
thymine. Guanine will only bond with cytosine. This is known as the "BasePair Rule." The bases can occur in any order along a strand of DNA. The
order of these bases is considered the genetic code. A strand of DNA
contains millions of bases.
Color all the deoxyribose sugars blue (light) ("D"). Color all the phosphates
pink (P).
Color
Color
Color
Color
the thymines orange.
the adenines green.
the guanines purple.
the cytosines yellow.
Note that that the bases attach to the sides of the ladder at the
sugars and not the phosphate.
The two sides of the DNA ladder are held together by their
bases, by hydrogen bonds. Hydrogen bonds connect the complementary
bases. The hydrogen bonds are represented by small circles. Color the
hydrogen bonds grey or black.
The DNA can actually "unzip" at the hydrogen bonds when
it needs to replicate - or make a copy of itself. DNA needs
to copy itself when a cell divides, so that the new cells
each contain a copy of the DNA
DNA Replication
Cells pass on their genetic code by replicating their
DNA. When DNA replicates, certain steps must be
followed correctly in order to prevent a mutation. Three
enzymes are used for the process. Helicase opens the
double helix up into two separate strands. DNA
polymerase reads the original DNA strand and lays down
2
Name ___________________________
Date
________ Block__________
complementary bases for each strand. Ligase glues the newly formed DNA
together, thus creating two separate strands of identical DNA.
Protein Synthesis
Transciption
It is the sequence of bases that determine which protein is to be made.
The sequence determines which proteins are made and the proteins
determine which activities will be performed. Because DNA is too big to go
through the nuclear pores a chemical called messenger RNA (mRNA) is
brought into the nucleus to interpret the DNA code. Like replication, the
DNA molecule unwinds and splits apart. The mRNA reads it, and like
replication, matches up complementary bases to the ones on the DNA. There
is one difference. RNA does not use the base thymine. Instead, a nitrogenous
base called uracil matches up with adenine. Another difference in RNA is
that it is single stranded, not double. In addition to that difference, mRNA
has the sugar ribose instead of deoxyribose. RNA stands for Ribonucleic
Acid.
Color the mRNA as you did the DNA, except Color the ribose a DARKER
BLUE, and the uracil brown.
Translation:
Once the mRNA has the coded strand
from the DNA it leaves the nucleus and
finds a ribosome. The bases on the strand
are broken up into groups of three. This is
known as a codon. There a molecule of
transfer RNA (tRNA) that matches with
complementary base pairs. Each tRNA is
grouped by 3 nucleotide pairs. At the top
of each tRNA molecule is an amino acid.
As a new tRNA arrives and binds to the
3
next codon on the mRNA strand, a peptide bond forms between the first
amino acid and so on, until it reaches a stop codon, which will indicate the end
of the amino acid chain. This chain of amino acids will create the template for
a protein.
Amino acid chart
The genetic code consists
of 64 triplets of
nucleotides. These triplets
are called codons. They
come from the mRNA.
With three exceptions,
each codon encodes for one
of the 20 amino acids used
in the synthesis of
proteins.
Amino acids are the
"building blocks" of the body derived from protein. Besides building cells and
repairing tissue, they form antibodies to combat invading bacteria and
viruses; they are part of the enzyme and hormonal system; they carry oxygen
throughout the body and participate in muscle activity. Eight are essential
(cannot be manufactured by the body) the rest are non-essential (can be
manufactured by the body). The codons are arranged in a chart, each which
code for a specific amino acid.
Note that for each table, the left-hand column gives the first nucleotide of
the codon, the 4 middle columns give the second nucleotide, and the last
column gives the third nucleotide.
4
Name ___________________________
Date
________ Block__________
Questions:
1. Write out the full name for DNA ________________________________________
2. What is a gene? _____________________________________________________
3. Where in the cell are chromosomes located? ________________________________
4. DNA can be found in what other organelles in the cell? ________________________
5. What two scientists are credited with establishing the structure of DNA?
___________________________________________________________________
6. What is the shape of DNA? ____________________________________________
7. The sides of the DNA ladder are composed of what? __________________________
8. The "rungs" of the DNA ladder are made of what? ____________________________
9. What sugar is found in DNA? In RNA? ____________________________________
10. How do the bases bond together?________________________________________
A bonds with _____
G bonds with _______
11. DNA is made of repeating units called ____________________________________
12. If the sequence of nitrogenous bases in one strand of DNA is GAGTC, what is the
sequence of bases in the complementary strand of DNA? ____________________
13. Why is RNA necessary to act as a messenger? Why can't the code be taken directly
from the DNA?________________________________________________________
14. Which of the nitrogenous bases pairs with uracil in an RNA molecule? _____________
15. Explain how codons determine the amino acid sequence of a protein.
___________________________________________________________________
16. An essential amino acid is an amino acid that _____________________________
17. Use the chart to determine which amino acids, the codons code for?
a) UCA ______________________
b) CAU_______________________
C) AAG ______________________
d) GCA _______________________
5
DNA Molecule
6
Name ___________________________
Date
7
________ Block__________