dna (4.3)
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Section 4.3 DNA
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Discovering DNA
• When did we first know that there was a nucleus in cells that contained large molecules called nucleic acids?
• mid-1800s
• By the 1950s we still were not sure how the nucleic acids and DNA were arranged.
Rosalind Franklin discovered that DNA is 2 chains of molecules in a spiral form.
Watson and Crick further studied the DNA model.
Watson and Crick discovered that each side of the ladder is make of a sugar-phosphate molecule.
What does DNA look
like
Each side of the ladder is make up of sugar-phosphate molecules.
Each molecule consists of a sugar called deoxyribose and a phosphate group.
The rungs of the ladder are made up of other molecules called nitrogen bases.
DNA has 4 kinds of nitrogen bases - adenine, guanine, cytosine, and thymine.
The nitrogen bases are represented by the letters A, G, C, and T.
Adenine always pairs with thymine, and guanine always pairs with cytosine.
Before Mitosis or Meiosis, DNA needs to be copied.
How does the double stranded DNA copy its information?
Copying DNA to prepare for Mitosis or Meiosis
Copying DNA to prepare for Mitosis or Meiosis
Copying DNA to prepare for Mitosis or Meiosis
Copying DNA to prepare for Mitosis or Meiosis
Copying DNA to prepare for Mitosis or Meiosis
• Most of your characteristics, such as the colour of your hair, your height, and how things taste to you, depend on the kinds of proteins your cells make.
• DNA in your cells stores the instructions for making these proteins.
• Proteins build cells and tissues or work as enzymes.
• The instructions for making a specific protein are found in a gene.
GEnes
GEnes
• A gene is a section of DNA on a chromosome.
• A chromosome contains 100’s of genes.
GEnes
• Proteins are made of amino acids linking together.
• The code for making a protein is found...
• .......in a gene.
• The gene determines the order of the hundreds or thousand of amino acids that link together.
• If you change the order, you make a different protein or nothing at all.
• Genes are found in the nucleus, but proteins are made on ribosomes in the cytoplasm.
• How does the code for a protein make it out of the nucleus to a ribosome?
• The codes for making proteins are carried from the nucleus to the ribosome by another type of nucleic acid called ribonucleic acid, RNA.
GEnes
RNA - Ribonucleic Acid
RNA is made in the nucleus on a DNA pattern but is different from DNA. If DNA is like a ladder, RNA is like a ladder that has all its rungs cut in half.
nitrogenbases sugar shape
DNA A G C T deoxyribose ladder
RNA A G C U* ribose ladder cut in half
*U - uracil
DNA vs RNA
1. An enzyme splits a DNA molecule, so that a gene can be copied. The gene is the instructions for how to make a protein.
1
Transcriptioncopying a DNA to make RNA
2. The free floating nitrogen bases in the nucleus match with a nitrogen base on the split DNA. The partnering is the same as when DNA is copied except that Adenine matches with Uracil.
2
Transcriptioncopying a DNA to make RNA
3. The nitrogen bases pair up on the split DNA temporarily until a complete gene is copied.
3
Transcriptioncopying a DNA to make RNA
4. The newly made mRNA will now detach from the DNA and leave the nucleus. 4
Transcriptioncopying a DNA to make RNA
mRNA rRNA tRNA
messenger ribosomal transfer
travel out of nucleus to ribosome
make up ribosomes bring amino acids to ribosomes
3 types of RNA
Translation - RNA to Protein
• Protein production begins when mRNA moves into the cytoplasm. There, ribosomes attach to it.
• Ribosomes are made of rRNA.
• Transfer RNA molecules in the cytoplasm bring amino acids to these ribosomes.
• Inside the ribosomes, 3 nitrogen bases on the mRNA temporarily match with 3 nitrogen bases on the tRNA.
• The same thing happens for the mRNA and another tRNA molecules.
• The amino acids that are attached to the two tRNA molecules bond.
• This is the beginning of a protein.
Translation - RNA to Protein
• The code carried on the mRNA directs the order in which the amino acids bond.
• After a tRNA molecule has lost its amino acid, it can move about the cytoplasm and pick up another amino acids just like the first one.
• The ribosome moves along the mRNA. • New tRNA molecules with amino acids match up and add
amino acids to the protein molecule.
Translation - RNA to Protein
3 nitrogen bases on mRNA temporarily match to 3 bases on the tRNA.
Another tRNA bonds.
The amino acids that are attached to the tRNA bond, beginning to form the protein.
Controlling Genes
• In many-celled organisms, each cell uses only some of the thousands of genes that it has to make proteins.
• Genes that code for muscle proteins will not be used in nerve cells.
Controlling Genes
• Cells must be able to control the genes by turning some off and some on.
• This is done in different ways:
• DNA is twisted so tightly that no RNA can be made.
• Chemicals bind to the DNA so that it cannot be used.
Mutations
• If DNA is not copied correctly the proteins might not be made correctly.
• Mutations - any permanent change in the DNA sequence of a gene or chromosome of a cell
• Examples: • cells receive an extra or are missing a
chromosome• outside factors: X rays, sunlight, some chemicals
Results of Mutations
REsults of Mutations
• A mutation might or might not be life threatening.
• If the mutation occurs in the sex cell then all the cells of the new organism will contain the mutation.
• Most mutations are very harmful, but some can be beneficial.
• Beneficial?
• A plant with a mutation might cause it to produce a chemical that certain insects avoid, insects will not eat the plant.