Chapter 11: DNA and Genes

11.1 – DNA: The Molecule of Heredity

1. DNA stands for deoxyribonucleic acid

2. DNA contains all the genetic information that determines an organism's traits

3. DNA controls ALL cell activities by its production of proteins

DNA Makes Proteins

1. All living things contain proteins. Proteins are essential to all of life.

2. ENZYMES ARE PROTEINS!!

3. Enzymes control all the chemical reactions in an organism and are extremely important molecules in biology.

4. DNA contains all of the information needed to build all of the proteins for our bodies.

The Structure of DNA

1.DNA is made up of repeating subunits called nucleotides.

2.Nucleotides have 3 parts: a)A simple sugar

b)A phosphate group

c)A nitrogen base

The Structure of DNA

1. A nitrogen base is a carbon ring structure that contains one or more atoms of nitrogen.

2. There are 4 possible nitrogen bases in DNA

a) Adenine (A)

b) Guanine (G)

c) Cytosine (C)

d) Thymine (T)

3. Thus, there are 4 possible nucleotides in DNA

DNA Nucleotides

1. The amount of Thymine is always equal to the amount of Adenine

2. The amount of Cytosine is always equal to the amount of Guanine.

Who Discovered the Structure of DNA?

1. In 1953 James Watson and Francis Crick discovered that the structure of DNA is made of two chains of nucleotides joined together by the nitrogen bases in the form of a double helix

Double Helix DNA

1. Adenine always bonds with Thymine

2. Cytosine always bonds with Guanine

3. A-T

4. C-G

How does DNA carry information?

1. DNA carries information in the form of nucleotide sequences.

2. For example:

a) The sequence ATATGCTCTA carries different information from the sequence CGGTATTAAC.

3. Different combinations of the same 4 nucleotides creates a near infinite number of unique genes.

Replication of DNA

1. Recall that before a cell can divide via mitosis it must first copy its DNA during interphase

2. The DNA in a cell is copied in a process known as DNA replication

Steps of DNA Replication

1. Double stranded DNA is cleaved into two separate strands by the enzyme DNA helicase. DNA helicase “unzips” the DNA by breaking the bonds between the nitrogen base pairs.

2. Next, free floating nucleotides are matched to their complementary nucleotides through base pairing.

3. Another enzyme called DNA polymerase bonds these new nucleotides into a chain.

4. When the whole process is complete, there will be 2 copies of the DNA.

11.2 – From DNA to Protein

1.Proteins are made up of long strands of amino acids.

2.The sequence of nucleotides in a gene contains the information for assembling the amino acids that will eventually become a protein.

RNA

1. RNA is similar to DNA in that it is a nucleic acid.

2. RNA differs from DNA in 3 ways:

a) RNA is single stranded (it looks like half a zipper) and DNA is double stranded.

b) The sugar in RNA is ribose and in DNA it is deoxyribose.

c) RNA contains 4 base pairs like DNA but RNA has a base pair called Uracil (U) instead of Thymine (T). Uracil still bonds with Adenine (A) just like Thymine (T) does.

What is the role of RNA in the cell?

1. Making proteins is like making a car in a factory.

2. In order to make proteins, like making a car, you need 3 main things:

a) Instructions on how to make the protein

b) The parts necessary to make the protein

c) Workers to assemble the parts into a protein.

What is the role of RNA in the cell?

1. DNA provides the instructions for building the protein.

2. Proteins are made up of amino acids. Amino acids are the parts needed to make the protein

3. There are 3 main types of RNA that act as workers to interpret the instructions and assemble the amino acids into a complete protein

What is the role of RNA in the cell?

1. The 3 types of RNA are:

a) Messenger RNA (mRNA) – Brings the information from the DNA in the nucleus to the cytoplasm where protein synthesis occurs.

b) Ribosomal RNA (rRNA) – rRNA makes up ribosomes. Ribosomes are the organelle responsible for assembling amino acids into proteins.

c) Transfer RNA (tRNA) – transports specific amino acids to the ribosome for assembly.

Protein Synthesis Overview

1.Protein synthesis occurs in two steps:

a) Transcription

b) Translation

Transcription

1. Transcription is the process of making mRNA from a DNA template.

2. Transcription is similar to DNA replication except that a single stranded mRNA molecule is formed instead of a double stranded DNA molecule.

Transcription

1. mRNA needs to be transcribed because DNA can’t leave the nucleus, but protein synthesis takes place in the cytoplasm.

2. mRNA carries the DNA’s message to the cytoplasm and tells the ribosome which proteins to make.

The Genetic Code

1. There are 20 different amino acids (the building blocks of proteins) but there are only 4 different nucleotides in RNA.

2. It takes 3 nucleotides to code for an amino acid.

a) For example: the nucleotide sequence UUU will code for the amino acid phenylalanine to be placed in the protein.

The Genetic Code

1. Each set of 3 nucleotides in mRNA that codes for an amino acid is called a codon.

2. How many different codons are possible with 4 different possible nucleotides?

a) Answer: 43 = 64 different possible codons.

3. There are 64 possible codons but only 20 possible amino acids.

The Genetic Code

1. More than one codon can code for the same amino acid.

2. However, for any one codon, there can only be one amino acid.

3. Certain codons do not code for amino acids; they are instructor codons.

a) For example: UAA is a stop codon and tells protein production to stop at this point.

Amino Acid Codon Chart

Translation: From mRNA to Protein

1. The process of converting the information in a sequence of nitrogen bases in mRNA into a sequence of amino acids that make up a protein is called translation.

2. Translation takes place at the ribosome in the cytoplasm.

3. In eukaryotic organisms, the mRNA is made in the nucleus and then moves to the cytoplasm where translation takes place.

Translation (continued)

1. When the mRNA arrives in the cytoplasm, Ribosomes attach to it like clothespins to a cloths line.

2. For proteins to be built, amino acids must be brought to the ribosomes.

3. tRNA is the molecule for responsible for bringing the amino acids to the ribosomes.

Ribosome

Translation (continued)

1. Each tRNA molecule has three nucleotides that are complementary to the three nucleotides on the mRNA strand.

2. The nucleotides on the tRNA are called an anticodon because they complement a codon on the mRNA.

3. Each tRNA carries only the amino acid specified by the anticodon.

Translation (continued)

1. As the amino acids are bonded together, the ribosome slides down the mRNA and moves to the next codon.

2. Amino acids are added to the chain until a stop codon is reached.

3. As all of the amino acids are built into a chain, a polypeptide is formed.

4. This polypeptide will eventually be a protein with a specific job in the body.

Polypeptide