SEXUAL REPRODUCTION AND GENETICS CHAPTER 10 SEXUAL REPRODUCTION AND GENETICS

Chromosome, spindle fibers, nucleus, and nucleolus SECTION 10.1 – MEIOSIS MAIN IDEA – MEIOSIS PRODUCES HAPLOID GAMETES. All of the differences you see in the room, different hair color, eye color, ear shapes, is the result of 2 sex cells combining during sexual reproduction. QUESTION: How are the following cell parts involved in mitosis? Chromosome, spindle fibers, nucleus, and nucleolus

Ex: hair color, height or eye color CHROMOSOMES AND CHROMOSOME NUMBER Everyone has characteristics passed on to them from their parents. Each characteristic is called a trait. Ex: hair color, height or eye color The instructions for each trait are located on the chromosomes, which are found in the nucleus of cells. The DNA on the chromosomes is arranged in segments called genes that control the production of proteins. Each chromosome has 100’s of genes.

HOMOLOGOUS CHROMOSOMES Human body cells (somatic cells) have 46 chromosomes. Each parent gave you 23 chromosomes creating 23 PAIRS of chromosomes or 46 chromosomes. The chromosomes that make up a pair (the one from mom & the one from dad) are called homologous chromosomes Homologous chromosomes in body cells have the same length, same centromere position, and carry genes that control the same inherited traits. These genes each code for the same trait (ex: earlobe) but maybe not the same type of trait (earlobe)

HAPLOID AND DIPLOID CELLS Gametes are sex cells that have half the number of chromosomes. In humans the number of chromosomes in a gamete is 23. Each species has a different # of chromosomes The symbol n can be used to represent the number of chromosomes in a gamete. Cells with n number of chromosomes is called a haploid cell. Haploos means single. Fertilization is the process by which one haploid gamete (egg) combines with another haploid gamete (sperm) Fertilization creates cells that are 2n (one n chromosome from mom & one n chromosome from dad) 2n cells are called diploid cells

Gametes are formed during a process called meiosis. MEIOSIS I Gametes are formed during a process called meiosis. Meiosis is a type of cell division that reduces the number of chromosomes Meiosis is a reduction division. Mitosis maintains the chromosome #, meiosis reduces the chromosome # by half through the separation of homologous chromosomes. Cells begin as 2n, but create gametes with n number of chromosomes Meiosis occurs in the reproductive structures of organisms that reproduce sexually. Meiosis involves 2 consecutive cell divisions called meiosis I & meiosis II

INTERPHASE Just like with mitosis, cells that undergo meiosis also go through interphase as part of their cell cycle. While in interphase cells carry out various metabolic processes, including the replication of DNA and the synthesis of proteins

Creating sister chromatids PROPHASE I Cells entering prophase I, the replicated (copied) chromosomes become visible. Creating sister chromatids Homologous chromosomes (mom’s sister chromatid + dad’s sister chromatid) form during synapsis and are held tightly together. Can be called a tetrad. Crossing over occurs this time. Crossing over is a process when segments of chromosomes are exchanged between a pair of homologous chromosomes. Prophase I continues with the centrioles moving to the cell’s opposite poles, spindle fibers forming and binding to the sister chromatids at the centromere.

PROPHASE I

METAPHASE I 2nd phase of meiosis Homologous chromosomes line up at the equator of the cell. Spindle fibers are attached to the centromere of each homologous chromosome

ANAPHASE I During anaphase I homologous chromosomes separate. Sister chromatids are pulled to opposite poles of the cell Chromosome number is reduced from 2n to n when the homologous chromosomes separate.

Sister chromatids are still joined at the centromere TELOPHASE I Homologous chromosomes (sister chromatids) reach the cell’s opposite poles. Sister chromatids are still joined at the centromere Sister chromatids might not be identical from when it started because of crossing over. During telophase I, cytokinesis usually occurs pinching in the animal cell or forming cell plate in plant cells. May or may not go into interphase. If cell goes into interphase, DNA is NOT duplicated again.

Telophase I

MEIOSIS II Meiosis is only half done after meiosis I. Prophase II the sister chromatids condense and spindle apparatus forms Metaphase II the sister chromatids line up at the equator by the spindle fibers.

MEIOSIS II Anaphase II the sister chromatids are pulled apart at the centromere by the spindle fibers and the chromosomes move toward the opposite poles of the cell. Telophase II the chromosomes reach the poles and the nuclear membrane and nuclei reform. At the end of meiosis II, cytokinesis occurs creating 4 haploid cells each with n number of chromosomes

MEIOSIS II

MEIOSIS PROVIDES VARIATION Genetic variation is produced during crossing over and during fertilization when gametes randomly combine. Depending on how chromosomes line up at the equator, 4 gametes with 4 different combinations of chromosomes can result.

SEXUAL REPRODUCTION VS. ASEXUAL REPRODUCTION Asexual reproduction the organism inherits all of its chromosomes from a single parent and are genetically identical to the parent. Ex: bacteria Sexual reproduction allows variation and beneficial mutations to accumulate faster in the population.