Genetics and DNA: The Science of Heredity
Ms. De Los Rios
7th Grade
12-1 The Genetic Code
Essential Question:
Vocabulary
Nitrogen bases- molecules that contain
nitrogen and other elements.
DNA replicationBefore a cell divides,
the process in which
DNA copies itself.
My Planet Diary pg. 408 – DNA Debut
In 1951, English scientist Rosalind Franklin discovered that DNA could
exist in a dry form and a wet form. Franklin made an image of the wet
form of DNA by exposing it to X-rays. The X-rays bounced off the atoms
in the DNA to make the image. The image was so clear that it helped
scientists understand the structure of DNA for the first time, and showed
how genetic information is passed from parent to offspring. Franklin’s
contribution to science was not only in her research, but also in that she
succeeded at a time when many people thought women shouldn’t be
scientists.
What does the x-ray
of DNA look like to you?
What Forms the Genetic Code? Pg. 409
It took almost 100 years after the discovery of DNA for scientists to
find out that it looks like a twisted ladder.
James Watson and Francis Crick published the structure of DNA in 1953
DNA contains the
genetic information
for cells to make
proteins. Proteins
determine a variety
of traits, from hair
color to an organism’s
ability to digest food.
The Structure of DNA pg. 409
•Parents pass traits to their offspring through chromosomes.
• Chromosomes are composed mostly of DNA, which is shaped like a
twisted ladder, or “double helix”. The sides of the double helix are made
up of sugar molecules called deoxyribose, alternating with phosphate
molecules.
•DNA’s full name, deoxyribonucleic acid, comes from this structure.
The Structure of DNA pg. 409
The rungs of DNA are made of nitrogen bases, molecules that
contain nitrogen and other elements.
adenine (A)
thymine (T)
guanine (G)
cytosine (C).
A gene is a section of a DNA molecule that contains the code for one
specific protein.
That code is a series of bases in a specific order—for example,
ATGA CGTA C.
A single gene may contain several hundred to a million or more
bases.
The Genetic Code- pg. 409
Genetic Structures
What is the order from largest to smallest?
Chromosomes, Genes and DNA Bases- pg. 410
The picture below shows the relationship among
chromosomes, genes, and DNA.
Because there are so many possible combinations of bases and
genes, each individual organism has a unique set of DNA. DNA
can be found in blood samples, however, because white blood
cells do contain DNA.
Humans have between 20,000 and 25,000 genes on their
chromosomes. Which base always pairs with cytosine? ____________
Gene
Deoxyribose
(a sugar)
Chromosome
Gene
Phosphate
Adenine
Cytosine
Thymine
Guanine
Apply it! Pg .410
Can you help solve the crime?
Someone robbed a jewelry store. The robber’s DNA was extracted from
skin cells found on the broken glass of a jewelry case. The police
collected DNA samples form three suspects. The letters below
represent the sequences of nitrogen bases in the DNA. Based on the
DNA found at the crime scene, circle the DNA of the guilty suspect.
Robber: GACCAGTTAGCTAAGTCT
Suspect 1: TAGCTGA
Suspect 2: GACGAGT
Suspect 3: CTAAGTC
1. Explain- Why can you solve crimes using DNA?
2. Infer- Could the police have used blood or the broken glass to test for
DNA? Why or why not?
Order of the Bases pg. 411
The code each gene contains determines the structure of a protein.
The order of the nitrogen bases along a gene forms a
genetic code that specifies what type of protein will be
produced.
Remember that proteins are long-chain molecules made of
individual amino acids.
In the genetic code, a group of three DNA bases codes for one
specific amino acid.
For example, the three-base sequence CGT (cytosineguanine-thymine) always codes for the amino acid alanine.
The order of the three-base code units determines the order
in which amino acids are put together to form a protein.
Assess Your Understanding pg. 411
1a. Identify- These letters represent the nitrogen bases on one strand
of DNA: GGCTATCCA. What letters would form the other strand of
the helix? ___________________________________________
b. Explain- How do parents pass traits such as eye color to their
offspring? ______________________________________________
_______________________________________________________
I get it! Now I know that the genetic code of nitrogen bases specifies
______________________________________________________.
How Does the DNA Copy Itself? Pg. 412
When a cell divides, it forms two daughter cells. Daughter cells need a
complete set of DNA to survive, so DNA copies itself. DNA replication is the
process in which an identical copy of a DNA strand is formed for a new cell.
Without DNA replication, daughter cells could not carry out their life functions.
What are the missing bases on the strands of DNA?
How Does DNA Copy Itself? Pg. 412
DNA replication begins when the two sides of a DNA molecule unwind
and separate, like a zipper unzipping, between the nitrogen bases. Next,
nitrogen bases in the nucleus pair up with the bases on each half of the
DNA.
Because of the way the nitrogen bases pair up, the order of
the bases in each new DNA strand exactly matches the order in the
original DNA strand.
Adenine always pairs with Thymine
A
T
Guanine always pairs with Cytosine.
G
C
Assess Your Understanding pg. 411
2a.Review- The (nitrogen base pattern / number of genes / size of DNA)
determines how DNA is replicated.
b. Describe- Where in the cell does DNA replication take place? ______
_________________________________________________________
_________________________________________________________
c. Challenge- What do you think would happen if the DNA code in a
daughter cell did not match the code in the parent cell. _____________
_________________________________________________________
_________________________________________________________