dna ls 5.3. what is dna? ■deoxyribonucleic acid –the hereditary material ■this is what you get...
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DNALS 5.3
What is DNA?
■ Deoxyribonucleic Acid– The hereditary material
■ This is what you get from your parents, and what is passed to offspring during reproduction
■ A type of nucleic acid (recall 4 main biological molecules)
■ A characteristic of all living things– The smallest bacteria has DNA
■ DNA is very similar between organisms
– Evidence for common ancestry
■ Contains the instructions for the organism – Called genes
DNA, Chromosomes and Genes
■ DNA is the hereditary material– Usually long and stringy (chromatin)
■ Wraps around proteins during cell division (chromosomes)
■ A segment of DNA that contains instructions is called a gene
Discovery of DNA
■ People knew something existed that passed traits on, but didn’t know what it was.– Mendel called them factors– Darwin said something would be
found that explains how traits are passed
■ In the early 1900’s, DNA was suspected to be the hereditary material
■ Rosalind Franklin and Maurice Wilkins – Took X-ray photographs of DNA
■ James Watson and Francis Crick– Took Franklin and Wilkins’ work
and discovered the structure of DNA
Structure of DNA
■ DNA is a chain of smaller units– Like links on a chain– Looks like a twisted ladder
■ Called a double helix
■ The smaller units are called nucleotides
■ A nucleotide is made up of 3 parts– The backbone is made of:
■ A sugar, called deoxyribose
■ A phosphate group
– The interior (rungs of the ladder) have:■ Nitrogen bases
Nitrogen Bases
■ 4 of them– Adenine (A), Thymine (T), Guanine (G), Cytosine (C)– Their sequence determines the gene
■ Bases come in pairs– They are held together by bonds
■ They give the DNA molecule its shape
– A-T (apple tree)– C-G (car garage)
DNA Replication
■ Occurs whenever the cell splits (mitosis: recall S-phase)
■ DNA unzips (just like a zipper) and splits apart– Each ½ provides a template to form another identical molecule
■ Example: Find the complementary strand– A G T C G A – T C A G C T
RNA
■ Another nucleic acid
■ Ribonucleic Acid
■ Similar to DNA, but key differences– Where DNA has 2 sides, RNA is single-sided– Where DNA has deoxyribose as its sugar, RNA has ribose– Where DNA has Thymine (T), RNA replaces it with Uracil (U)
■ T DOES NOT EXIST IN RNA!
Types of RNA
■ mRNA (messenger RNA)-carries message from nucleus to ribosomes
■ rRNA (ribosomal RNA)-RNA in a ribosome that reads the code
■ tRNA (transfer RNA)-carries amino acid to ribosome
Making a protein■ Proteins are chains made of units called amino acids
– The sequence of amino acids determines the protein’s shape■ The shape determines the job
■ The sequence of amino acids is determined by the gene– Better definition for gene: a segment of DNA that codes for a specific
protein
■ mRNA is made from a strand of DNA (Called transcription)– Practice: What RNA strand would form from this DNA strand?
■ A T G C G T A
■ U A C G C A U
– Because it is small, mRNA can leave the nucleus■ It travels to a ribosome, where rRNA bonds to it
■ tRNA carries amino acids to the ribosome, and place them in the correct spot (translation)– The amino acids bond together, forming the protein
Central Dogma of Biology
■ Dogma-an idea known to be true that is central to the field
■ DNA RNA Proteins Traits
Mutations
■ Change in nucleotide sequence on DNA
■ Different types– Nucleotides can be deleted– A wrong nucleotide can be added– The wrong nucleotide can replace the correct one– Chromosome parts can be broken
■ Can be natural, or can be the result of a mutagen (something in the environment that causes a mutation)– Smoking, alcohol, disease, excess sunlight, asbestos, etc.
Results of Mutations
■ Most mutations are neutral– You’d never even know it happened
■ The cell repairs it, or it occurs in noncoding (junk) DNA
■ Other mutations are bad– Can lead to disease, such as cancer
■ Occasionally, mutations are beneficial– Give something to the offspring that they did not previously have
■ Example: Making an animal’s coat look a little more similar to its surroundings
– This is what natural selection acts on (more to come next chapter)
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