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)