Young Scientist Program Curriculum

Reading Assignment 2: Nucleic Acids

Background:

Experiments show that traits are passed on in ways that can be predicted using math as you have seen with the worm experiment. Other experiments showed that all living things are made of cells that arise from a single cell. If one stops to think about what this means, a reasonable answer is that all cells must contain instructions that are telling it what to do and these instructions are handed down from the first cell that was formed.

Inside every cell is a structure called the nucleus (it's the large round object inside the cells in the drawings of cells). The nucleus is a hollow structure that contains the cell's instructions so it could be considered the cell's command center. Inside this nucleus is a long molecule called DNA. Packages of DNA are present in structures called chromosomes which you have heard about in the previous sections. The DNA in the chromosomes is the chemical that encodes the cells instructions. If one unwinds the strand of DNA one can see that DNA is composed of 2 very long chains of four chemicals called bases which people abbreviate A, C, T, and G. These four letters form a code which the cell understands.

Certain lengths of these letters tell the cell to do something or make something and these regions that have these instructions are called genes. The genes enable the single cell to become a complete organism, and the differences in certain genes make a human become a human and a worm become a worm. The number of genes a living thing has depends on the complexity of the creature. Humans have about 100,000 thousand genes but C. elegans has only about 10,000 genes!

Vocabulary:

Define the following terms in your own terms:

Nucleus:

DNA:

Chromosomes:

Genes:

Question:

1. Why do humans have 100,000 genes but an animal like C. elegans only 10,000?

What do the genes in DNA tell the cell?

The DNA in every cell contains all the information, or the master plans, needed for that cell to live. Every gene contains directions for building a specific protein. Proteins are chains of molecules called amino acids which must be assembled in a particular order for the protein to do its job in the cell. If you think of DNA as a master plan for a building, proteins are the structural and functional elements like the walls and wiring that actually make up the building. Some examples of proteins include hemoglobin in blood, enzymes for food digestion, and antibodies which fight infection.

There are 20 amino acids used to make all the thousands of different proteins that a living organism needs. To make things simple, scientists use single letters to stand for each of the 20 amino acids.

How does the information in DNA get transmitted to the rest of the cell?

DNA is the master copy of information for the cell and copies of the genes in it are made for daily use. These copies are called RNA and the process of converting the information contained in DNA into RNA is called transcription. One major difference between RNA ad DNA is that RNA can leave the nucleus of a cell and enter the cytoplasm where the protein-building machinery is located while DNA remains in the nucleus. Since RNA can carry the information from DNA to the other parts of the cell, the master plan contained in DNA never has to leave the nucleus and the cell still has the directions it needs. Why might it be a good idea to make copies of DNA so that this master plan can be preserved? Three good reasons for making RNA are:

1.
2.
3.

RNA, like DNA, consists of chains of four nucleotides represented by the letters A, U, C, and G. The A, C, and G nucleotides in DNA are represented by the A, C, and G nucleotides, respectively, in RNA, but the T base in DNA is represented by the U base in RNA.

DNA nucleotide

A
C
G
T

RNA nucleotide

A
C
G
U

How is the information in DNA and RNA stored and read?

How does the cell figure out what information is contained in DNA when DNA is such a long complicated molecule? The answer to this question is that a code is created by the order in which the four nucleotides or bases (A, T, C, G) are arranged. When an RNA copy is made from DNA, the four nucleotides in RNA are assembled in the exact same order with the exception that U is used in place of T. How can 4 nucleotides be used to make a code for 20 amino acids? Let's consider some options:

1. Each nucleotide represents one amino acid.
   (Four nucleotides = four amino acids)
2. Combinations of two nucleotides represent one amino acid.
   (Eight combinations)
3. Combinations of three nucleotides represent one amino acid.
   (64 combinations)
4. Combinations of four nucleotides represent one amino acid.
   (256 combinations)

Yes/No

It turns out that different combinations of nucleotides in groups of 3 represent the different amino acids. These arrangements of nucleotides are called codons because they encode the information needed to make a protein.

The chart below is the genetic code used by the cells in your body to translate the sequence of your nucleotides into the proteins needed for life.

GENETIC CODE
1st position	2nd position				3rd position
	U	C	A	G
U	F	S	Y	C	U
	F	S	Y	C	C
	L	S	Stop	Stop	A
	L	S	Stop	W	G
C	L	P	H	R	U
	L	P	H	R	C
	L	P	Q	R	A
	L	P	Q	R	G
A	I	T	N	S	U
	I	T	N	S	C
	I	T	K	R	A
	M	T	K	R	G
G	V	A	D	G	U
	V	A	D	G	C
	V	A	E	G	A
	V	A	E	G	G

Note: Some codons do not translate into amino acids but instead send a stop signal which marks the end of a protein. Some amino acids can also be encoded by more than one codon.

Using the code above, translate the following sequences of codons into a sequence of amino acids.

1.

Amino Acids: ___ ___ ___ ___ ___ ___ ___ ___

Codons: AUG. AUC. AGA. GCC. UGU. CUC. GAA. UAG.

2.

Amino Acids: ___ ___ ___ ___ ___ ___ ___ ___

Codons: AUG. UAC. AGU. ACC. GAA. CGC. UAU. UGA.

What happens if one nucleotide is changed or mutated?

Now translate the sequences:

1.

Amino Acids: ___ ___ ___ ___ ___ ___ ___ ___

Codons: AUG. AUC. AGA. GCC. UGA. CUC. GAA. UAG.

2.

Amino Acids: ___ ___ ___ ___ ___ ___ ___ ___

Codons: AUG. UAC. AGU. ACC. GUA. CGC. UAU. UGA.

These words no longer make sense because they are cut off in the middle or are misspelled. In a similar way, mutations in DNA can result in a protein that does not function properly. In sickle cell anemia, for example, one amino acid in hemoglobin has been changed and has a big effect on how the hemoglobin functions. The mutant worms that we have seen also look different from the normal worm or move differently all because proteins, encoded by DNA, have not been made of the normal amino acids.