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Pennsylvania State University
Human Development and Family Studies
HD FS 129
Sally Vavala

HDFS 129 Online – Narrative Chapter Two 1 Use these notes in addition to reading the text, to be able to fill in the term lists. In this review, if I think something is adequately covered in the term list or the chapter, I won’t go over it here. This class is not a biology class, so we will be somewhat simplistic. There are three types of cells, body cells, germ cells, and gametes. Body cells make up most of our bodies – skin, muscles, blood, etc. Each body cell has 23 pairs of chromosomes in its nucleus, or 46 chromosomes in all. Chromosomes – coils of DNA that look like rod-shaped structures in the cell nucleus that store and transmit the genetic information in the form of genes. The chromosomes occur in pairs, one of each pair coming from the mother, and one of each pair coming from the father. Genes – segments of the coils of DNA – units of hereditary information – molecules that tell cells and protein how to develop – made up of deoxyribonucleic acid, or DNA DNA is a chemical substance in the form of a double helix, or like a twisted ladder or spiral staircase. The genes are the short segments of this staircase. The DNA ladder can come apart down the middle – as if you cut each rung of the ladder in the middle. Then each part of the ladder can reproduce itself, so that you end up with two identical ladders instead of the original one. All body cells divide by mitosis. Mitosis is a process of cell division where each chromosome duplicates itself completely. In mitosis, both chromosomes of every pair split lengthwise down the middle (the DNA splitting as described above), and each part replicates itself to become complete again. Then the chromosomes and their replicas move to opposite sides of the body cell, and then the cell splits down the middle, creating two new cells. Each of the two new cells has exactly the same chromosome content that the original cell had. This process of mitosis goes on and on in the body cells throughout the course of development, forming muscle, bone, organs, etc. Germ cells – The germ cells are cells specialized for reproduction. At first they divide by mitosis, but then they switch to dividing by meiosis. Meiosis is a type of reduction cell division that only germ cells go through. The germ cell splits into two cells, but the chromosomes themselves do not split and do not replicate themselves. HDFS 129 Online – Narrative Chapter Two 2 In meiosis, you start out with a germ cell containing the full complement of chromosomes, 23 pairs, or 46 in all. Instead of each single chromosome splitting itself like in mitosis, during meiosis the pairs of chromosomes just line up down the middle of the cell, and then just separate and move away from each other. Then the germ cell divides down the middle, with half of its chromosomes having gone into one of the resulting cells, and the other half of the chromosomes having gone into another of the resulting cells. These new cells, that have been formed by the process of meiosis of a germ cell, each only have half of the original number of chromosomes – the number of chromosomes has been reduced by half, cutting it from 46 in the germ cell, to 23 in each of the resulting cells. That is why meiosis is called reduction cell division. These new cells that have only 23 chromosomes each, are called gametes. The female gamete is the ovum, or egg. The male gamete is the sperm. Because meiosis occurs when the germ cells form the gametes, for a new human being to be made with all the genetic material with the normal number of chromosomes in all of its body cells (46), two gametes have got to unite. So at conception, the sperm with 23 chromosomes from the father unites with the ovum (egg) with 23 chromosomes from the mother, creating a new cell (a future baby) with 46 chromosomes – the 23 pairs needed to program all of development. How is sex of the baby determined? The sex of the baby is determined by one pair of chromosomes, called the sex chromosomes. This pair is just one of the 23 pairs of chromosomes we all have. In the female, both chromosomes of this pair are large, and called X chromosomes. Female - XX In the male, one chromosome of this sex chromosome pair is a large X chromosome, but the other one is a smaller Y chromosome. Male – XY Because in a female the germ cell always has two X chromosomes for the sex chromosome pair, when the germ cell goes through meiosis and forms the gametes (eggs), each egg or ovum can only get an X chromosome. Each ovum is therefore only carrying genetic information to form a girl. Because in a male the germ cell has one X chromosome and one Y chromosome for the sex chromosome pair, when the germ cell goes through meiosis and forms the gametes (sperm), the sperm can get either an X or a Y chromosome. Some sperm are therefore carrying genetic information to form a girl, and some of the sperm are carrying genetic information to form a boy. HDFS 129 Online – Narrative Chapter Two 3 If a sperm carrying a Y chromosome unites at conception with an ovum carrying an X chromosome, the resulting baby will be XY, or a boy. If a sperm carrying an X chromosome unites at conception with an ovum carrying an X chromosome, the resulting baby will be XX, or a girl. The man’s sperm determines the sex of the baby. Genotype – the actual genetic make-up of an individual. You cannot see a person’s genotype just by looking at him. Phenotype – the observable way the genotype is expressed – like being tall, having dark hair and brown eyes, and being intelligent Different genotypes may produce the same or diff
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