Chapter 3: Reproduction and Heredity Lesson 1: How do organisms Reproduce and grow? (Pages A78-A85) Main Idea: Living things pass their genetic traits to offspring through asexual or sexual reproduction. Every species lives on by reproducing. To reproduce is to make more of one’s own kind. Asexual Reproduction: reproduction in which ONE parent produces offspring that are identical to the parent. Example: Bacteria through bacteria fission. Hydra through budding. Some plants through growing from roots, buds, or cutting. Sexual Reproduction: reproduction from the union of TWO parents that each pass on traits to the offspring. Each organism in every generation gets a unique combination of traits. Example: Humans Some plants through forming seeds. In flowering plants, the flowers produce both eggs and sperm. Pollen is the source of the sperm cells. The egg cell is located deep in the flower, inside the ovule. After fertilization, the ovule develops into a seed. The seed can then germinate and grow into a plant. This new plant has the genetic traits inherited from its parents, but it’s genetically different from them. Fertilization: the process in which a male gamete joins a female gamete to produce a new cell that develops into an organism. Gametes: specialized reproductive cells. They are the end product of a process that divides a parent cell twice. A male gamete is called sperm. A female gamete is called an egg. Large organisms grow from one fertilized egg cell into an adult with perhaps trillions of cells. This happens because cells divide over and over again, often becoming more specialized during the process. Different types of cells include: skin cells, red blood cells, white blood cells, muscle cells, & nerve cells. Each cell contains a huge amount of genetic information. When cells divide, they pass the genetic plan for the entire organism to the new cells. Different parts of this plan are expressed in the different cells. The genetic information is held in molecules called DNA. A cell’s DNA is stored in the nucleus in structures called chromosomes. Mitosis: the stage in the cell cycle during which the nucleus divides. The Cell Cycle: All body cells grow and divide in a regular cycle called the cell cycle. STAGE 1: The longest part of the cell cycle is the Interphase. During Interphase, the cell spends much of its time growing. It also makes a copy of each of its chromosomes. STAGE 2: Mitosis, the division of the nucleus. During Mitosis, the cell gets ready to divide. It begins organizing the pairs of chromosomes in its nucleus. Then, the pairs separate into two nuclei. Each new nucleus receives one copy of each chromosome, which is why the daughter cells are identical to their parent. STAGE 3: (FINAL STAGE): Cytokinesis. In this stage, a new cell wall or cell membrane divides into two daughter cells. The daughter cells then enter their own cell cycles. The genetic information in a cell controls the cell’s growth, development, and reproduction, creating genetic traits. Genetic Traits: characteristics of living things; those that genes control. In species that reproduce sexually, gametes typically combine only with gametes of the same species. Over time, a species develops variations. These are the different forms of a trait. For example, blue and brown are variations of eye color. Chromosomes come in pairs. Each member of a pair of chromosomes carries genes from one parent. One chromosome came from the father and the other chromosome came from the mother. Dominant Trait: is produced by a variation of a gene that is always expressed. Recessive Trait: is one that will not appear if the gene for the dominant trait is present. Selective Breeding: two individuals with the desired traits are mated to each other. Lesson 2: How do organisms Inherit Traits? (Pages A86A97) Main Idea: An organism’s genes determine its inherited characteristics. Gregor Mendel – honored as the father of genetics. Mendel wondered why the pea plants he grew in his garden had different physical traits. Some were tall, some were short, some produced yellow seeds, and others produced green seeds. Mendel also noticed that new pea plants often looked similar to their parents, but not always. Sometimes offspring looked different. Mendel chose to study pea plants because they grow quickly and have many traits that are easy to observe. Mendel experimented to find out what would result from crossing or mating two plants with different traits. Mendel crossed two plants, one was tall and one was short. Mendel called the parent plants the “parental generation” or the “P generation.” He called the first generation of offspring the “first filial generation” or “F1 generation.” Mendel predicted that some of the offspring would be tall and some would be short. Mendel’s prediction was wrong. All the offspring grew to be tall. What happened to the shortness trait? To find out, Mendel let the plants in the F1 generation produce lots of offspring. He called the offspring of the F1 plants the “F2 generation.” Again, Mendel was surprised. The plants in the F2 generation were a mix of short and tall even though their parents (the F1 plants) were all tall. Mendel found that ¾ of the F2 plants were tall and ¼ were short. Put another way, for every 3 tall plants, there was 1 short plant. That is a 3:1 ratio. Mendel expanded his experiment to include other traits, such as color and shape. In all his tests, he got the same 3:1 ratio of results in the F2 generation. ¾ of the F2 plants had one trait and ¼ had the other trait. When Mendel studied peas, genes hadn’t been discovered yet. Mendel proposed that “factors” control traits. Mendel reasoned that these factors must come in pairs, one from the male and one from the female. Based on his observations, Mendel inferred that one factor covers up the other. For example, all the F1 plants were tall, so tallness covers up shortness. Mendel’s conclusions proved correct. Scientists later found that his “factors” are genes which often come in different forms. Allele: one of different forms of a gene for a trait. Each pea plant inherits two alleles for a given gene, such as the gene for height. Individual alleles control the plant’s traits. So, it’s really alleles that are inherited, not traits. In Mendel’s F1 pea plants, the allele for tallness covered up the allele for shortness. When this happens, scientists say that one allele is dominant and the other allele is recessive. A dominant allele is expressed even if it is paired with a recessive allele. On the other hand, a recessive allele is expressed only if the organism has two recessive alleles. In other words, a dominant allele always covers up a recessive allele. If tallness is a dominant trait, why did short plants appear in Mendel’s F2 generation? The answer lies in the alleles. Even though all the F1 plants had a dominant trait, each plant still carried an allele for the recessive trait. Thus, some pea plants in the F2 generation got 2 recessive alleles, one from each parent. As a result, these F2 pea plants were short. Genetic traits shorthand: Instead of writing the description of a trait, scientists use a letter to represent the gene for a trait such as height. A capital letter is used for the dominant allele. Example: for tallness, T A lowercase letter is used for a recessive allele. Example: for shortness, t If a plant inherits 2 alleles for shortness, the letters tt represent the alleles. If a plant inherits 2 alleles for tallness, the letters TT represent the alleles. If a plant inherits an allele for tallness and an allele for shortness, the letters Tt represent the alleles. Purebred strains: TT or tt; meaning their parents were either tall or short exclusively. By crossing the purebred strains, each plant in the F1 generation received one allele for tallness and one for shortness. Such plants are called hybrids. Hybrid: an organism that has two different alleles of the same gene. A hybrid receives a dominant allele from one parent and a recessive allele from the other parent. Which allele is recessive and which is dominant in the abbreviation Bb? Predicting traits: Probability is the chance that a certain event will happen. In genetic studies, probability is usually stated as a ratio. It can also be written as a fraction or as a percent. If we toss a coin, the coin has 2 sides, before we toss it, you do not know whether it will land on one side or the other. When you call “heads” or “tails” you are making a prediction. Because a coin has 2 possible outcomes, the probability of either outcome is 1 out of 2. Written as a ratio, the probability as 1:2. Written as a fraction, the probability is one half (½). Written as a percent, the probability is 50%. All of the values represent the same ratio. The probability of receiving an allele from a parent is a lot like flipping a coin. Each parent has 2 alleles for a gene, so the probability of receiving either allele is 1/2. From two parents, the probability of receiving a specific pair of alleles is ½ x ½, or ¼. For example, in Mendel’s F2 cross of pea plants, each offspring had a ½ probability of receiving the shortness (t) allele from one parent, and a ½ probability of receiving the same allele from the other parent. The result is that ¼ of the offspring received both shortness alleles. Punnett Squares One way of finding the probability of each result is to use a chart called a Punnett Square. A Punnett Square helps a researcher predict each possible combination of alleles that may occur in the offspring of a cross. The allele that each parent will pass on is decided by chance. The different alleles that can come from each parent are shown above and beside the square. Each box receives the letter above it and the letters beside it. Each box in the Punnett Square represents an offspring. Look at the Punnett Squares. In one, all the offspring are gray. This is because eac one got one dominant allele (G) and one recessive allele (g). In the second Punnett Square, notice that 3 of the boxes have gray rabbits and 1 of the boxes has a white rabbit. The probability of having gray fur is 3:4 and the probability of having white fur is 1:4. These ratios are the same ones that Mendel saw. In the F2 generation, 3 in 4 pea plants were tall and one in four was short. Inheritance Patterns: Many alleles are either dominant or recessive, but that is not always the case. In certain species of chickens, for example, feather color is inherited differently. When a black chicken is crossed with a white chicken, some of their chicks have both black and white feathers. This is an example of codominance. Codominant: result of two alleles both being expressed. In other cases, a dominant allele is only partly expressed. Incomplete dominance: result of a dominant allele being only partly expressed. For example, the pink color of some four o’clock flowers comes from a dominant allele for red that is only partly expressed. How can you determine the probable ratio of traits in offspring? Phenotype: the physical appearance an organism presents to observers. Genotype: the sum total of all the genes that an organism inherits. Heterogeneous: different in kind; mixed unevenly. Homogeneous: the same; uniform in composition; mixed evenly. Meiosis In organisms that reproduce asexually, offspring receive the same genetic information as the parent. Because there is only one parent, there are no different alleles to combine. In organisms that reproduce sexually, however, each of two parents gives only half their chromosomes to the offspring. Gametes (eggs & sperm) each have half the number of chromosomes as the body cells in the same organism. The process that produces gametes is called meiosis. Meiosis: cell division that reduces the number of chromosomes by half. During meiosis, the parent cell divides twice and the gametes have copies of half the parent’s chromosomes. When the gametes unite, the new cell has two complete sets of similar chromosomes. One set came from the father and the other set came from the mother. Scientists now understand that Mendel’s “factors” are the alleles of genes. Genes are located on chromosomes, and chromosomes are what cells pass to offspring. What are genes made of? Genes contain DNA. DNA (deoxyribonucleic acid): the genetic material of all living things. DNA contains a code that can be copied and that allows it to send “messages” to the cell and direct its activities. What type of cells form by meiosis? Human Genetics: Humans have 46 chromosomes, which occur in 23 pairs. Each chromosome contains a huge number of genes. In fact, many scientists think that every cell has approximately 40,000 genes. These genes are spread out among 23 pairs. In humans and many other animals, chromosomes are classified into two groups. Two of the chromosomes are called sex chromosomes. They are abbreviated X and Y. Males are XY (one of each type of chromosomes in their cells). Females are XX (no Y’s). Fraternal twins receive different sets of genes from their parents. They are not identical. The differences include different sex chromosomes. Thus, one twin is a girl and the other is a boy. All other chromosomes are called autosomes. A pair of autosomes resemble each other much more than X and Y chromosomes do. The genes an individual receives determines many things about his or her traits. They determine gender, eye and hair color, and many other traits. Some genes also put an individual at risk for health problems. Studying how genes are passed in humans can help doctors and scientists treat and counsel people for these problems. Your genetic traits are only part of what makes you who you are. As you grow older, the knowledge and skills you learn and the choices you make help change and define you. What is one possible benefit of understanding human genetics? Lesson 3: How is Genetic Information Used? (Pages A100A109) Main Idea: DNA is the material that makes up genes and determines traits. The structure of DNA is important to the way living things use and pass on genetic information. In 1953, scientists James Watson and Francis Crick built the first model of DNA and proposed an explanation of how it worked (their model relied partly on the work of Rosalind Franklin and Maurice Wilkins). Their model showed, DNA has the shape of a double helix. This looks like a spiral staircase or twisted ladder. The sides of the ladder are made up of sugars and phosphates. The steps are the base pairs. Base pairs contain the coded information that DNA holds. DNA uses 4 different bases: Adenine (A); thymine (T); guanine (G), and cytosine (C). When they pair up, they ALWAYS pair up in the same way. Adenine pairs with thymine, and guanine pairs with cytosine. Adenine (A) - Thymine (T) Guanine (G) - Cytosine (C) A single gene on a chromosome may have anywhere from a hundred base pairs to a million base pairs. You can read a single strand of DNA as a very long word. In fact, the word would be billions of letters long! A single strand of DNA is made of only four letters: A, T, G, and C. The arrangement of these letters forms the long coded message that DNA contains. Scientists have studied the DNA of a huge variety of Earth’s living things, from the largest animals and plants, to the smallest protists and bacteria. With the exception of some very ancient and unusual species, every organism uses DNA in the same way. All use DNA of the same shape, the same 4 bases, and the same four bases, and the same type of coded information. Who contributed to the discovery of DNA? When cells copy their DNA to pass it on to offspring we call this process replication. Replication: the process of making identical copies of DNA. How does this happen? The answer depends on 2 of the DNA molecule’s features: 1. Its double helix 2. The way its bases pair up Replication begins when the two sides of the double helix “ladder” separate, much like a zipper coming unzipped. This breaks the bonds that hold two base pairs together. Remember, adenine pairs only with thymine, and guanine pairs only with cytosine. On each side of the ladder, the exposed single bases find a new partner from unpaired bases in fluids around them. As the bases pair up, a new sugar-phosphate side forms and a new ladder results. In this way, each side of an original DNA molecule is used as a template to make a new DNA molecule. The new molecules have exactly the same sequence of base pairs as the original. Each new DNA molecule may become part of a new cell or even a new organism. DNA stores instructions for cell structure and function. In order for these instructions to be carried out, the DNA must be transcribed, or decoded, into RNA. The information can then be used to build a cell structure or carry out a cell function. RNA (ribonucleic acid) is similar to DNA, with 2 important differences: 1. RNA has the base uracil instead of thymine. Uracil pairs with adenine. 2. RNA is a single strand, not a double helix. The RNA made by transcription is used to make proteins. These proteins determine the structures of a cell and the functions they will perform. Insulin, for example, is a protein that helps keep the level of sugar in the blood fairly constant. Making Proteins: Proteins are made outside of the nucleus of a cell on structures called ribosomes. Ribosome: a cell structure where proteins are manufactured. Before the process can begin, the cell needs a messenger to take the genetic code from the DNA inside the nucleus to the ribosomes. The messenger is a kind of RNA, called messenger RNA, or mRNA. In the nucleus, DNA serves as a pattern from which mRNA is made. Again, the DNA molecule’s “ladder” separates between the base pairs. This time, RNA bases pair up with them to form a single strand of mRNA. The strand of mRNA then separates from the DNA. Information from the DNA has been transferred to the mRNA strand. After mRNA leaves the nucleus, it attaches to a ribosome. The ribosome holds an mRNA strand so that three bases at a time are in position to bind to another form of RNA. This other form of RNA is called transfer RNA, or tRNA. Transfer RNA molecules have an amino acid attached to them. Amino acids are building blocks of proteins. There are about 20 different kinds of tRNA molecules. Each carries one of the 20 different kinds of amino acids. Each tRNA unit binds to an mRNA strand by linking three of its base to three bases in the mRNA. The order of the three mRNA bases is a code for one amino acid. One tRNA with its amino acid binds to every three bases along the mRNA strand. In this process, the amino acids with the tRNA line up next to each other. Bound together, a chain of amino acids forms a protein or part of one. The chain grows as building blocks are added until a three-base code that means “stop” is reached. Then, the ribosome releases a new protein. How do the roles of messenger RNA and transfer RNA differ? Changes in DNA: Imagine that instead of base G, the base A is added to the DNA molecule. This will cause a permanent change in the sequence of bases in the DNA. When such a change occurs, a cell’s genetic information may change. This change may affect only one gene or perhaps many genes. Any permanent change in a gene or a chromosome is called a mutation. Mutation: any change in a genome or a chromosome. Gene mutation changes one gene or a few. Chromosomal mutation can affect a large amount of genetic information. Sometimes a mutation may only affect a certain cell. Other times, a mutation happens in a cell that divides to make gametes. In this case, the mutation may be passed on to an offspring. A mutation can cause the proteins that an organism makes to be different. With different proteins, some of the organism’s physical traits will be different. Mutations can be harmful or beneficial. Some mutations change not just the protein itself, but also how much protein is made. The mutation may also change where the protein is made. These changes in DNA can result in too much or too little protein being made, or they can result in proteins being made in the wrong cell at the wrong time. Scientists now know that many disorders in humans are caused by mutations. Some forms of cancer are caused by mutations. For example, exposure to ultraviolet causes mutations in skin cells. This may lead to skin cancer. Sickle-cell anemia is a genetic disorder caused by a mutation in the gene for hemoglobin. Hemoglobin is a protein that helps red blood cells carry oxygen through the body. The mutation causes the red blood cells to be sickle-shaped. These cells can get stuck on blood vessels, causing pain and tissue damage. Some mutations are very helpful. For instance, one chromosomal mutation causes strawberry plants to make very large fruits. The cells of these strawberries have extra sets of chromosomes. Beneficial mutations can make a living thing more likely to survive and reproduce. One example is the resistance of bacteria to antibiotics. Gene mutations help some of the cells become immune to the deadly effect of an antibiotic. How is a chromosomal mutation different from a gene mutation? Studying DNA: No two people have the same fingerprints. Unless you have an identical twin, no one has the same DNA pattern as you. Investigators of crimes often take DNA samples of skin and hair and use them to identify a person. A DNA fingerprint is made to show the unique patterns in an individual’s DNA. Scientists who study DNA have also been working to figure out the entire human genome. A genome is all the genetic information that is found in the members of a species. Figuring out the entire human genome would help doctors and scientists better understand how our bodies develop and work. It will also lead to a new understanding of disease and help scientists develop new medical treatments. How is DNA like a fingerprint?