Obviously, I’ve taken a bit of a break from updating the site, as I’ve taken a step away from the classroom for the last year or so.
Apparently, however, tons of people are in need of science help, as indicated by the over 5,000 (and counting!) visitors this site received in the last month.
That got me thinking – there’s no reason this site should be put on hold because I’m not extending it to a class in need…I can serve as a virtual teacher for those who’d like some help.
So, then, to the point: Do you need help with a science topic not on the site? Do you need a site topic explained a little further? Is there something specific you’d like to see on the site? Need an idea for a science fair project? Looking for an experiment to wow your parents/teacher/class with?
If so, use the contact form located here, and shoot me an email. I’m glad to help, create new content, or whatever it is that the community might need.
On this site, you can review key terms, print a neatly formatted set of vocabulary flashcards, test yourself, randomly rearrange and review flashcards, etc. Seriously, it will help.
Also, if you sign up (for free) on this site, it will keep track of your top score on flashcard reviews, and compare it to those of your classmates. Competition anyone?!?
The wonderful part of genetics is that, with a basic understanding of gene dominance and probability, one can (fairly reliably) predict the offspring that are possible from a genetic pairing.
The interactives below provide a great review of this process. Part 1 reviews Mendelian genetics, and Part 2 reviews Punnett Squares. Enjoy (and learn!).
Part 1: Mendelian Genetics and Dominance
Part 2: Punnett Squares, etc.
Punnett Square Class Documents:
Click the image to the left to download Punnett Practice sample items.
Click the image to the left to download the SpongeBob Genetics worksheet.
Click the image to the left to download our Genetics With A Smile class project.
Introduction to DNA
All animals and plants on Earth share something in common: they all use the same code to grow and function.
This code can be found in bacteria, elephants, oak trees, and humans. It’s the “master plan” for all organisms, like the source code for a computer program. It’s the reason your toes are on the ends of your feet (where they belong) instead of on your hands or hanging off the edges of your ears.
This code is called DNA, which is shorthand for Deoxyribose Nucleic Acid. DNA exists as long molecules within the nucleus of almost every cell in an organism.
DNA is in the form of a twisted ladder that scientists call a double helix.
The rungs of the ladder make up the four-letter DNA code or alphabet: A, T, C, and G.
These alphabet pieces are called bases and they bond together (in base pairs) according to special rules: A always pairs with T, and C always pairs with G [A–T C–G].
When the DNA code is read by the cell, only one strand of the code is used. The string of letters (or bases) make 3-letter long “words” and the words make “sentences,” just like the letters, words, and sentences in your science textbook. In DNA, the “sentences” are called genes. One strand of DNA may contain many genes.
Genes tell the cells how to make other complex molecules called proteins. Cells produce thousands of different proteins that work together to allow organisms to do all the functions necessary for life.
Click here for an in-depth tour of DNA from University of Utah Genetics
Fun Fact #1:
Each cell in our body contains a lot of DNA (about 1.7 meters). If our cells were enlarged to the size of an aspirin pill, the DNA in that cell would be 10,000 meters long. That’s as long as 109 football fields.
Fun Fact #2:
If you opened up all your cells and laid out the DNA end to end, the strands would stretch from the Earth to the Moon about 6,000 times.
Chromosomes and Mitosis
The DNA in each cell is coiled up tightly and packed into compact units called chromosomes.
Each body cell of an organism has the same number of chromosomes.
The number of chromosomes in each cell is different for different species of organisms. In humans, every cell contains 46 chromosomes. Dogs have 78 chromosomes in every cell. Cats have 38 chromosomes, horses have 64, and fruit flies have 8.
Within the nucleus of each cell, the chromosomes are found in homologous pairs, with half coming from each parent. So for humans, 23, or half of the 46 chromosomes in each of your cells came from your father and half (23) came from your mother.
Fun Fact #3:
The least number of chromosomes is found in a species of ant that has only 2 chromosomes. The record for the most chromosomes goes to a species of fern, which has 1,260 chromosomes in each cell.
Remember that within the nucleus of every cell in the body, the chromosomes contain the DNA, which carries the genetic code that directs how every cell functions. When the body makes new cells as part of growth, or to repair damaged tissues, the cells divide in a process called mitosis.
During mitosis, each parent cell divides to form two daughter cells.
The chromosomes in each parent cell also divide, so that each daughter cell has an exact copy of the chromosomes found in the parent cell. This way skin cells, for example, will all have the same genetic information so they will all function alike.
Asexual and Sexual Reproduction
Some organisms can produce “offspring” by making exact copies of the parent.
This is called asexual reproduction, and is found in single-celled organisms like the Paramecium and Amoeba, and also in simple organ-isms like sponges. Many plants can also reproduce asexually. Very few complex organisms use this method of reproduction (see an exception below.)
The offspring produced by asexual reproduction are always exact copies of their parents — in effect they are all clones of the parent organism. Asexual reproduction allows an organism to make lots of copies of itself, but all the copies are identical.
So, how do we get so much variation in nature? Why don’t we all look just like our brothers and sisters and parents?
When most organisms produce offspring, they use a different method, called sexual reproduction.
In sexual reproduction, offspring are not exact copies of their parents. Instead, they receive half their genes from their mothers and half from their fathers, so offspring may have characteristics in common with both parents, but they also have many unique traits—characteristics that may not be seen in either parent.
During sexual reproduction, special cells called gametes are produced by each parent.
Because these cells undergo a special type of cell division called meiosis, each gamete contains only half the number of chromosome found in the parent’s body cells.
So for humans, each body cell has 46 chromosomes (or 23 pairs–half from mom and half from dad), but each gamete only has 23 chromosomes, or one chromosome from each pair. During sexual reproduction, the male and female gametes unite to form a zygote, which then has the 46 chromosomes characteristic of human body cells.
Zygotes undergo cell division (mitosis) and grow and eventually develop into adults. However, offspring produced by sexual reproduction are not identical to their parents or to their brothers and sisters, because they each received different combinations of the chromosomes from each parent. This is one way that variation is introduced into each generation.
Another way that variation can be achieved is through mutation.
Sometimes when chromosomes are making copies of themselves, a mistake in the DNA code can occur. Also, there are factors in the environment that can cause errors or changes in the DNA code, such as UV radiation from sunlight, ionizing radiation from natural or man-made radioactive substances, and harsh chemicals introduced into the environment. All of these can damage DNA and may result in a mutation in a gene.
All living things are made up of tiny structures called cells.
A cell is the smallest unit that can perform all the things we need to be able to live – like taking in energy, etc.
Because of their small size, however, cells were not discovered until microscopes were invented in the 1600s. A scientist and inventor named Robert Hooke was the first person to describe plant cells by looking at corks, plants and fungus under a high-powered magnifier (kind of like a primitive microscope).
Later, Anton Van Leeuwenhook (pronounced: Lay-vun-hook) invented the microscope we think of today (much more powerful than Hooke’s) and observed animal blood cells, which were different in different types of animals. He was also the first person to see protozoa.
After about another 200 years of observations based on the discoveries of Hooke and Leeuwenhook, scientists were able to conclude that cells existed in all living things, and eventually came up with something called the Cell Theory.
The Cell Theory
all organisms are made of cells
the cell is the basic unit of all living things
all cells come from other cells
So that’s the history of the cell in a very, very brief amount of info.
Now, on to the specifics of cells:
All cells, regardless of type – be they human, plant or bacterial cells – have a few parts in common:
Cell Membrane:a protective layer that separates the cell’s inner contents from the world around it. Covers and protects the cell. It also controls the flow of objects into and out of the cell.
Cytoplasm: fluid that exists inside the cell and helps support the cell.
DNA: genetic information that is used to make new cells and organisms.
In some cells, DNA is kept inside of the nucleus.
There are two basic kinds of cells: prokaryotic cells and eukaryotic cells. Within each category, there are even more subcategories of cells – but in general, they are separated by one thing; prokaryotes have no nucleus, eukaryotes have a nucleus.
Prokaryotes
Prokaryotes are single celled organisms with no nucleus or membrane-bound organelles.
Prokaryotes are bacteria.
They can be classified as archaebacteria or eubacteria.
Archaebacteria live in extremely hostile environments like swamps, volcanic pools and very salty areas.
Eubacteria are common bacteria. They exist EVERYWHERE!
Eukaryotic cells
Eukaryotes are every other type of cell.
They have all the most basic parts of a cell, plus a few more:
Nucleus: an organelle that contains the cell’s DNA and that has a role in growth, metabolism, and reproduction
Membrane-bound Organelles: Small “organs” in a cell’s cytoplasm that are specialized to perform specific functions
Eukaryotes can also be single-celled or multicellular.
When they are single-celled, they are called eukaryotic cells; when they are multi-cellular, they are called eukaryotes.
Plants, animals, fungi, etc are all eukaryotes.
