Living organisms are distinguished by their ability to reproduce their own kind Offspring resemble their parents They have similar characteristics

(Do you look exactly like your brother or sister?) Heredity – the transmission of traits from one generation to the next. (Do you look exactly like your brother or sister?) Even though there is similarity between offspring and parents, there is also variation Variation helps to make each species unique.

Inheritance of Genes Genes – the chemical factors that determine traits. Genes are segments of DNA They are located on chromosomes in the cells nucleus Humans inherit tens of thousands of genes from their parents. This accounts for family resemblances

Genetics is the scientific study of heredity.

Gregor Mendel Is known as the father of Genetics because he studied the transmission of traits from one generation to the next. He came up with several principles

Principles of Genetics A. Each organism must inherit a single copy of every gene from each of its “parents” B. When an organism produces is own gametes (sex cells), those two sets of genes must be separated from each other so that each cell contains just one set of genes. (we get only half of our chromosomes from each parent)

In animals and plants reproductive cells transmit the genes. Gametes (sex cells) – are the vehicles that transmit the genes. Females = egg cells Males = sperm cells

Chromosome Number In humans, each somatic cell (body cell)-any cell other than a sex cell, has 46 chromosomes.

Check for Understanding How is a gamete different from a somatic cell?

Any cell with two chromosome sets is called a diploid cell. 2 chromosomes after replication

These two sets of chromosomes are arranged in homologous pairs, meaning… One set is inherited from the mother, the other from the father. Homo- same Pairs have similar genetic information on them. Note, same size

Diploid is abbreviated 2N So human body cells (somatic cells) are diploid and (2N = 46) Skin cells Nerve cells

Gametes (sex cells), contain only a single set of chromosomes. These cells are haploid, which means “one-set”. Haploid is abbreviated (N) So human gametes are haploid and (N = 23)

Check for understanding If a cell is N=4 How many chromosomes would be in a body cell that is 2N? If a sex cell has 12 chromosomes, how many chromosomes would be in a somatic cell? If a body cell is 2N = 8 How many chromosomes would be in the animals sex cells? N=

How are haploid gametes produced from diploid cells?

Meiosis… an overview A reduction in the number of chromosomes. Diploid cells produce haploid gametes. Takes place where gametes are formed (in humans, in the ovaries and testes). Homologous chromosomes are separated. Each new cell produced has only one set of chromosomes. Involves two distinct divisions, called meiosis I and meiosis II. Allows for genetic variations.

Meiosis Meiosis Takes place in two sets of divisions, meiosis I and meiosis II Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half.

Meiosis I - Reduction Meiosis II-Division Reduces the number of chromosomes from diploid to haploid Meiosis II-Division Produces four haploid daughter cells

Division I – Meiosis I 6 chromosomes. 2 sets = diploid Homologous pairs line up across the center of the cell. Or pairs line up across the center Homologous chromosomes separate. Or chromosome pairs Sister chromatids remain attached Homologous chrom. Pair. Or chromosomes form pairs Cell prepares for division Specific type of body cell called a germ cell Form a structure called a tetrad. Which is 4 chromatids of a homologous pair Crossing over occurs. Which is the exchange of genetic information

Division II - Meiosis II Meiosis I – Con’t Division II - Meiosis II 2 cells are produced Similar to mitosis Chromosomes line up across the center of the cell Sister Chromatids separate and move to opposite sides 4 cells are produced Haploid or Diploid? Still Haploid cells

Sex Cell Formation Spermatogenesis- production of sperm Process takes about 74 hours. Males produce sperm. From one germ cell, 4 cells will be produced that will become sperm.

Sperm Development Males contain germ cells called spermatogonium within their testes. These cells will undergo both divisions of meiosis to produce sperm cells with half the number of chromosomes. This division begins when signaled by hormones. Men produce sperm throughout their life time.

Sex Cell Formation Oogenesis- egg cell production Takes 15-30 yrs. In females, the cell divisions at the end of meiosis I and meiosis II are uneven, so that a single cell, which becomes an egg, receives most of the cytoplasm. Cytoplasm contains nutrients and we want that egg to have lots of nutrients. The three other cells do not form eggs and are called polar bodies Oogenesis- egg cell production You tube : nursereview.org animation on oogenesis Miracle of Human Life 14min

Division I – Meiosis I You must also know all the other happenings in the cell that were similar to mitosis Interphase Prophase I metaphase I anaphase I Cell prepares for division (homologous chromosomes) pair Form a structure called a (tetrad) -which is (4) chromatids (crossing over) occurs - which is the (exchange) of genetic information (homologous pairs) line up across the center of the cell (homologous pairs) separate (sister chromatids) remain attached Meiosis I is the reduction phase because the chromosome number is reduced by half

Division II - Meiosis II Meiosis I – Con’t Division II - Meiosis II (2) Cells are produced Cells are Haploid Nuclear envelope breaks down Spindle fibers form Centrosomes move (chromosomes) line up across the center of the cell (sister chromatids) separate and move to opposite sides (4) Cells are produced Cells are Hapolid Meiosis II is the division phase because we end up with a total of four cells