Mr. Harwood
Updated: Feb 2015
Course: SNC2D1
Unit: biology
Lesson 8,9: Title: DNA
Apparatus needed: molecules kit
Preliminaries: do quiz
collect any missing HW
p4: finish onion/cheek cell lab
Lesson:
What is DNA?
Every cell contains DNA in the nucleus. DNA tells the cell what to do (how?). Each plant or animal has its own DNA that turns it into a cat or a cactus.
Each person has small variations in human DNA that makes their DNA unique – this is used for forensics.
DNA in a cell is split up into pieces, not just one long molecule. Each piece is called a chromosome. All human beings have 46 chromosomes.
You inherit 23 chromosomes from each parent.
DNA (or chromosomes) have sections called genes that control different traits (eg. height, skin colour)
verbal discussion of how chromosomes make someone male or female. Also about why you can’t marry a close relative.
DNA stands for deoxyribonucleic acid. It is a very long thin molecule shaped like a spiral staircase (double helix). The DNA of each human cell is about 2-3 m long (if all the chromosomes are in 1 line).
Why is it in the nucleus? The nucleus protects the DNA from the rest of the cellular activities. The nucleus only looks after the DNA and makes RNA from it.
Why is it doubled? To allow other molecules to read the DNA and correct errors in it. (There are two copies of the data that are zipped together. This is different from having a second chromosome from the other parent.)
DNA stores information in a code using molecules that are called bases. There are 4 of these molecules and we assign them letters based on their names: A, C, G, T. (for adenine, cytosine, guanine, thymine)
Human DNA has about 3 billion bases. We can only decode about 10% of our DNA.
Why is it in a spiral? The outer “backbone” protects the inside where the information is from damage by chemical reactions.
Day 2:
What does DNA look like? How is the DNA molecule made?
DNA is always a right hand spiral (like a normal screw thread).
What DNA is made of
It is made up of six subunits as seen in the diagram above, right. The spiraling sides or backbones of DNA are not smooth ribbons like the diagram above: they are made up of alternating sugar (deoxyribose) and phosphate units.
The rungs of the ladder are made up of pairs of (nitrogenous) bases. (These always connect to the sugar unit.) There are four bases: A= adenine, T = thymine, G = guanine, and C = cytosine. Aand T always pair up together, so do Cand G.
DNA in all organisms is made up of the same six subunits. The only thing that differs is the order of the bases (or rungs in the ladder). The order of the bases is what encodes all of our genetic data.
DNA is very stable when it is in the nucleus. The bases are protected inside the DNA molecule, while the -P-S-P-S- backbones are exposed to the aqueous environment of the cell. (The bases on RNA are exposed to the cell, so RNA is not a permanent storage for genetic code.) If the hydrogen bonds between DNA bases were exposed to water, they would come apart.
Now: in groups of 4, build A, C, T, G. Also deoxyribose. See reference handout for structure.
What are the two things that are done with DNA?
- make an exact copy of it (for cell division)
- copy one section (a gene) to RNA to make protein.
DNA is copied to RNA inside the nucleus.
RNA then leaves the nucleus and goes to the ribosome to make protein.
Show virus videos.
RNA (ribonucleic acid)
RNA is the same as DNA except
- It is only single stranded
- It uses ribose instead of deoxyribose in its backbone.
(Ribose has an extra OH group which prevents RNA from being wound into stable double helixes) - It uses the base uracil (U) instead of the base thymine (T).
(U is easier to make, but not as stable as T. Using U would introduce errors into DNA.)
It is important that DNA is double stranded because it is much more stable than a single stranded molecule would be. The double stranded structure of DNA allows for error correction and repair.
It is important that RNA is single stranded so that it can be read by ribosomes. RNA doesn’t have to be extremely stable. It is destroyed after a few hours and new RNA is made from DNA when needed.
diagram of how it all fits together (the large hand drawn one)
Video of HIV infecting a cell
Homework:
Evaluation: