12-1 The Genetic Code

Essential Question:

Genetics and DNA: The Science of Heredity Ms. De Los Rios

7th Grade

Vocabulary

Nitrogen bases- molecules that contain

nitrogen and other elements.

DNA replication-

Before a cell divides,

the process in which

DNA copies itself.

My Planet Diary pg. 408 – DNA DebutIn 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 StructuresWhat 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? ____________

Chromosome

Gene

Gene

Deoxyribose(a sugar)

Phosphate AdenineCytosine

GuanineThymine

Apply it! Pg .410Can 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: GACCAGTTAGCTAAGTCTSuspect 1: TAGCTGASuspect 2: GACGAGTSuspect 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. 411The 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 (cytosine-guanine-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? ______________________________________________

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I get it! Now I know that the genetic code of nitrogen bases specifies

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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 the DNA Copy Itself? Pg. 412

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? ______

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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. _____________

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