- Slides: 56
DNA Structure and Function
What is DNA? What do the letters DNA stand for? ◦ Deoxyribonucleic acid Where is DNA located? ◦ Prokaryotic cells: in the cytoplasm ◦ Eukaryotic cells : in the nucleus How many pieces of DNA are in cells? ◦ Prokaryotic cells: one, circular piece of DNA ◦ Eukaryotic cells: several, linear pieces of DNA (46 in humans)
Write what you think… What is a chromosome? What does a chromosome look like?
Structural Forms of DNA A. Prokaryotic cells: circular in form
Structural Forms of DNA B. Eukaryotic cells: DNA can be found in two different structural forms: 1) chromosome: ◦ tightly packed strands of DNA found in a dividing cell. ◦ Usually shown as a doubled chromosome, or X-shape You have __46______ chromosomes in your body cells. Each piece is about __5______ cm long. What is the total length of DNA in one body cell? 3_ meters! (9 ft) How does that much DNA fit inside each of your cells? ◦ Tied around histones Histones are proteins which act like thread spools. DNA is wound around the histones in order to fit inside a cell.
Structural Forms of DNA B. Eukaryotic cells: DNA can be found in two different structural forms: 2) chromatin: ◦ Strands are loose and exposed ◦ Code for proteins “recipes” can be read.
Structural Forms of DNA Why have 2 different forms of DNA in your cells? ◦ The advantage of chromosomes is _condensed genetic material can easily be moved to new cells during cell division. ◦ The advantage of chromatin is __loose genetic material can be ‘read’ because the DNA code is exposed. Each one of your body cells has an exact copy of DNA. That includes toe cells and nose cells, ear cells and eye cells!
Gene: _a segment of DNA that codes for the production of a protein.
Chromosome Number Every species of eukaryotic organisms have a specific number of chromosomes in each cell: Organism Number of Chromosomes in each cell. goldfish 94 chicken 78 horse 64 sand dollar 52 chimpanzee 48 dog 48 human 46 brown bat 44 corn plant 20 housefly 12 fruit fly 8 Why are these numbers so different? 1) ____chromosomes come in different sizes__ 2) __different organisms have different numbers of genes_
What is a Gene? Gene: 1) A unit of hereditary information which can be passed on to future generations. 2) A segment of DNA on a chromosome which 3) Genes hold the code for synthesis of holds the code or ‘recipe’ for a protein. ◦ proteins : a functional group of many amino acids ◦ Polypeptides: a smaller, non-functional group of amino acids (part of a protein) ◦ traits or parts of a trait : physical, behavioral and physiological traits
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What is a Gene? Review of the amino acid structure of proteins: Circle the R groups to identify individual aa’s. How many different KINDS OF amino acids could be used to form a single protein or polypeptide? __20, one for each R group
The Structure of DNA Which group of organic compounds does DNA belong to? nucleic acids Considering that DNA is a long molecule that must periodically be copied, it must be composed of simple units that fit together easily. DNA is made of repeating subunits called nucleotides. Each nucleotide is made up of 3 parts: ◦ a)_sugar_______ ◦ b) __phosphate group (PO 4) ◦ c) _nitrogen base_____ The sugar, __deoxyribose__and the phosphate groups are identical in all DNA nucleotides but there are four different types of nitrogen bases: ◦ __adenine (A)_________ ◦ __guanine (G)________ ◦ __thymine (T)________ ◦ ___cytosine (C)_______
Structures of the Molecules That Make up a Nucleotide: 1) Deoxyribose Sugar (ribose sugar minus an oxygen molecule) vs. Ribose Sugar (found in RNA)
Structures of the Molecules That Make up a Nucleotide: 2) Phosphoric Acid
Structures of the Molecules That Make up a Nucleotide: 3) One of four different nitrogen bases: PURINES ◦ Adenine: ◦ Guanine: PYRIMIDINES ◦ Thymine: ◦ Cytosine:
NUCLEOTIDE: A nitrogen base bonded to a sugar and a phosphate group. Here is an example of an adenine nucleotide:
As you can see, the chemical structure of a nucleotide is difficult to draw so we normally draw a nucleotide using symbols/letters to represent the parts. In the diagram above, what does the "A" stand for? ____adenine___ What other letters could be used in that position? T, C, G In the diagram above, what does the "S" stand for? _sugar_ In the diagram above, what does the "P" stand for? phosphate group How many different types of nitrogen bases are there in DNA? ___4____ Name them: ___adenine, thymine, guanine, cytosine_ If each nucleotide contains _one__ nitrogen base. How many different types of DNA nucleotides can be formed? _four NOTE: Another type of nucleotide, Uracil, will replace the Thymine nucleotide in RNA molecules.
The drawing below is a MODEL representing a short piece of a DNA molecule. The key identifies phosphates, sugar and nitrogen bases. The bonds that hold the molecules together in DNA are strong, covalent bonds. The bonds holding the nitrogen bases are NOT covalent but are weak, hydrogen bonds.
Circle a nucleotide on the drawing above.
Circle a nucleotide on the drawing above.
What does each nucleotide have in common? ___sugar, PO 4__ How do nucleotides differ from each other? ____different nitrogen base_ Notice that each nucleotide bonds with a nucleotide above and below it. This forms a strand of DNA. How many strands make up a molecule of DNA? 2 The two strands are then twisted into a spiral shape, double stranded structure called a double helix.
Look at the model of DNA on the previous page. Use a red, blue, yellow and green colored pencil and follow the directions to answer the questions: Look at drawing A. Circle a nucleotide on the right hand side or strand of the DNA molecule. What three parts can you identify? 1. ___sugar 2. __phosphate (PO 4) 3. __nitrogen base__ List the names of the different bases: __A= adenine, G= guanine, C= cytosine, T= thymine__ Color the circled nucleotide. Color sugar red and the phosphate blue. What is the name of the nitrogen base that you have circled? Color the bases as follows: adenine and guanine-yellow, cytosine and thyminegreen. Circle another nucleotide on the left hand strand of the DNA molecule. Color the sugar, phosphorous and nitrogen base as above. Imagine that the molecule of DNA in drawing A looks like a ladder. What two molecules make up the sides of the ladder? ___sugar__ and ___phosphate group What two molecules make up the rungs of the ladder? (Rungs are where you place your feet. ) ____one nitrogen base_ and a second, complimentary nitrogen base_ Color the remainder of the DNA molecule in drawing A according to the directions.
At what point are the right and left strands of DNA nucleotides joined? _between nitrogen bases Is there any order to the pairing of the nitrogen bases down the center of the DNA molecule? ◦ adenine is hydrogen bonded to thymine ◦ cytosine is hydrogen bonded to guanine Is this a pairing of purines with pyrimidines? ___purine is bonded with pyrimidine_______ Is there any order to the nucleotides down the right hand strand of the DNA molecule? __no__ Is there any order to the nucleotides down the left hand strand of the molecule? ___no____
In Summary: In a molecule of DNA, adenine is always paired with __thymine T___ In a molecule of DNA, guanine is always paired with ___cytosine C_ In a molecule of DNA, a purine is always paired with a ___pyrimidine We call this system of base pairing __complimentary base pairing___ If the sequence of nucleotides in one strand of a DNA molecule is What would be the complementary base pairs on the other strand? T-A-C-G-G-T-C-A-A A-T-G-C-C-A-G-T-T Describe the shape of the DNA molecule in drawing B. twisted ladder We call this twisted, double strand shape a ___double helix___
A segment of DNA is composed of 100 base pairs. If 25 of the nitrogen bases are adenine, how many are thymine? a. 15 b. 20 c. 25 d. 30 e. 35 Human DNA contains about 10% cytosine. thymine? a. 10% b. 20% c. 40% What percent of the bases are d. 80% e. 90%
Chromosomes can be seen in a single or double-arm, replicated form Chromatid Centromere Two configurations of chromosomes: 1) Single arm chromosomes are composed of a single chromatid. 2) Double-arm chromosomes are made up of paired, genetically identical chromati called sister chromatids. Sister chromatids are joined at the centromere. Because sister chromatids are formed during replication of DNA, they are identical right down to the nucleotide sequences! Sister Chromatids
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Complete the blanks showing the "central dogma" of molecular biology. DNA --> ______________ --> ________
Question 1 What process is illustrated by the arrow below? A: DNA duplication B: DNA diploidy C: DNA replication D: DNA mutation
Who Discovered the Structure of DNA? James Watson and Frances Crick (1953): along with Maurice Wilkins, were awarded the Nobel Prize for determining the structure of DNA.
Who Discovered the Structure of DNA? Rosalind Franklin (1951): Expert in X-ray Crystallography, her work was used by Watson to determine the structure. Died before the prize was given out —cannot be awarded posthumously.
The Human Genome Project Genome : The entire set of genes that make up the DNA of an organism. Completed in 2003, the Human Genome Project (HGP) was a 13 -year project coordinated by the U. S. Department of Energy and the National Institutes of Health. During the early years of the HGP, the Welcome Trust (U. K. ) became a major partner; additional contributions came from Japan, France, Germany, China, and others. Project goals were to: identify all the approximately 20, 000 -25, 000 genes in human DNA, determine the sequences of the 3 billion chemical base pairs that make up human DNA, store this information in databases, improve tools for data analysis, transfer related technologies to the private sector, and address the ethical, legal, and social issues (ELSI) that may arise from the project. (Source: http: //www. ornl. gov/sci/techresources/Human_Genome/home. shtml)
DNA replication DNA unwinds and unzips from the enzyme DNA helicase. DNA polymerase helps to link free floating DNA nucleotides between complimentary nucleotides. Covalent Bonds Join nucleotides on the SAME strand Hydrogen Bonds between opposite strands. Each new DNA strand formed contains the SAME sequence of nucleotides.