7D63 Genes
Background Genes are too small to see with microscopes. So how did microscopes help scientists figure out where in the cell the genes are located? Scientists studied cells that were dividing to find evidence of Mendel’s ideas.
Chromosomes Every organism must make new cells, either to replace damaged cells or to reproduce itself. Around 1900, scientists saw that just before cell division the nucleus was not visible and small dark chromosome structures appeared. When the cells split apart the chromosomes divided evenly between the two new cells.
Stopping To Think 1 How is the function of cell division in single cell organisms different from cell division in multicellular organisms? Write a paragraph in your lab book and be prepared to discuss it.
Cell Division Chromosomes are usually long fine strands like very thin spaghetti packed in the nucleus. Before the cell divides it makes copies of all its chromosomes so that its two offspring cells can each get a complete set. Then the chromosomes become coiled, which makes them visible under a microscope. Finally the cell divides.
Copies Each human cell has 46 chromosomes. When a human cell divides, the two new cells each contain 46 chromosomes. How can 46 chromosomes becomes two sets of 46? Each crisscrossed chromosome is two identical copies that are attached to each other. A double-chromosome splits during division to make 2 identical but separate chromosomes.
Stopping To Think 2 What would happen to the number of chromosomes in each cell if copies of them were not made before cell division? Write a paragraph in your lab book and be prepared to discuss it with the class.
Male And Female The 46 human chromosomes can be sorted into 23 matching pairs, one from each parent. Each chromosome looks identical to its partner except pair 23, the sex chromosomes. Human females have 2 X-chromosomes, while males have one X- and one Y-chromosome. These all behave just like in Mendel’s model.
Sexual Reproduction Sex cells (sperm and egg) are formed by a special kind of cell division where each cell receives copies of only ½ of the chromosomes. In humans, 46 chromosomes are found in the body cells but egg & sperm only have 23. When a sperm fertilizes an egg to form a new zygote, that new cell has 46 chromosomes, half from the mother and half from the father. https://www.23andme.com/gen101/origins/
Stopping To Think 3 Why must the number of chromosomes in the sperm and egg be half the number of the chromosomes in the other organism cells? Write a paragraph in your lab book and be prepared to discuss it with the class.
Gene Pairs Think of the 23 pairs of chromosomes as 23 pairs of socks. An egg cell has one of each kind of sock and a sperm cell has the matching sock Only when they are together in fertilization does the resulting cell contain 23 complete pairs of socks, one of each pair from the male parent and the other from the female parent. The chromosomes have similar genes but different alleles for traits.
Stopping To Think 4 Would 2 children with the same 2 parents be expected to have the same chromosomes? Write a paragraph in your lab book and be prepared to discuss it with the class.
Many Genes Humans have about 30,000 pairs of different genes, but the cell nucleus has only 23 pairs of chromosomes. After careful study scientists determined which gene, or group of genes, is responsible for each specific trait. Complex traits have a greater number of different genes that contribute to it. Some cancers result from the combination of genes. https://www.23andme.com/gen101/genes/
DNA Genes are on long chromosome molecules called DNA (DeoxyriboNucleic Acid). Sometimes gene mutations (random mistakes) are made when DNA is copied to make a new set of chromosomes. If this happens during formation of a sex cell, the offspring will be affected. Some mutations don’t make much difference, some are helpful & some harmful.
Mutation A mutation is like a change in a word in a sentence. In the sentence “I hear that noise” the 4 letters in the word “hear” have meaning. What if we change one of the letter? “I heer that noise” looks funny but sounds the same & would mean the same if you heard it. “I fear that noise” changes the whole meaning Some mutations change the gene information.
D63 Gene Analysis: pix/paragraph Below is a flow diagram (a series of connected pictures) showing DNA, genes, chromosomes, and cells in a human body. Copy the diagram & write a paragraph to explain it listing each part, location and size, compared to the other parts.