Meiosis and Sexual Reproduction Chapter 6

Sexual Reproduction Sexual Reproduction – reproduction that requires two parents and produces offspring that are genetically different from each other and from either parent.

Sexual Reproduction Sexual reproduction, through the shuffling of DNA, produces genetic diversity. This variation in genetic information (DNA) in offspring produces individuals that may have advantages over another organism of the same species. Example – may increase survival in an organism because it will be better equipped to cope with changes in its environment.

Diploid Number Diploid Number (2n) – in eukaryotic organisms, chromosomes occur in pairs. Every single body cell (except in the egg and sperm) contains 46 chromosomes. In humans, the male and female contribute 23 chromosomes each resulting in 46 chromosomes in their offspring.

What are Gametes? Gametes – specialized sex cells necessary for reproduction. Females – egg Males – sperm Haploid Number (n) – half the diploid number is found in the egg and sperm cells.

sperm (23) + egg (23)  zygote (46) Fertilization Once fertilization takes place between a male sperm (n) and a female egg (n), the result is a zygote (2n). The zygote then undergoes mitosis and cell division and develops into an embryo. In humans: sperm (23) + egg (23)  zygote (46)

How do we produce Gametes? Meiosis Meiosis occurs in reproductive organs. Process to produce haploid cells called gametes. The number of chromosomes in the daughter cell is half that of the parent cell. Meiosis has similar stages to mitosis, but has two main phases, meiosis I and meiosis II. Metaphase I in particular is different between meiosis and mitosis.

Sperm Production vs. Egg Production

Homologous Chromosomes A pair of matching chromosomes are that are similar in function and height. There will be 23 pairs of homologous chromosomes in humans.

Meiosis Videos Mitosis vs. Meiosis Rap Video Amoeba Sisters – Meiosis Video

MEIOSIS: REDUCING CHROMOSOME NUMBER

Meiosis I – Prophase I Chromosomes coil and become visible. Nuclear membrane disappears. Spindle fibers form. Homologous chromosomes move towards each other and pair.

Meiosis I – Metaphase I Pairs of homologous chromosomes move to the midline of the cell. These pairs are aligned on either side of the metaphase plate.

Meiosis I – Anaphase I Homologous chromosome pairs are pulled away from each other towards opposite ends of the cell. Paired chromatids remain attached.

Meiosis I - Telophase I Paired chromatids may uncoil and become less visible. Nuclear membranes may form around each chromosome cluster. Spindle fibers disappear. Cytokinesis occurs.

Meiosis II Phase II of meiosis is the mitotic division of haploid cells. Although there will be four new non-identical cells at the end of meiosis II, all other processes for this division are similar to mitosis.

Meiosis II – Prophase II Paired chromatids coil. Spindle fibers form. Nuclear membrane disappears.

Meiosis II – Metaphase II Paired chromatids line up in the middle of the cell. The chromatids are attached to spindle fibers.

Meiosis II – Anaphase II Each pair of chromatids split at the centromere to form two independent chromosomes. Chromosomes move to opposite ends of the cell.

Meiosis II – Telophase II Chromosomes uncoil and become less visible. Nuclear membranes from around four new nuclei. Spindle fibers disappear. Cytokinesis occurs.

Meiosis II – Cytokinesis II Same process as in mitosis. Four non-identical cells are formed. These cells are called haploid cells (n). # chromosomes = half the # of chromosomes of parent cell.

MEIOSIS: REDUCING CHROMOSOME NUMBER

Meiosis Events Meiosis I Matching chromosome pairs (homologous chromosomes) move to opposite poles of the cell - two daughter cells result. Meiosis II Chromatids of each chromosome are pulled apart - the end result is four haploid cells, each with half the number of chromosomes. These develop into gametes.

Exchanging DNA Crossing Over (refer to page 192) In meiosis I, chromatids of chromosome pairs can cross over each other and exchange DNA segments - this increases genetic possibilities and produces more variation. Independent Assortment The pairs of chromosomes in meiosis I separate independently, creating many different combinations of chromosomes in the daughter cells.

Differences in Mitosis and Meiosis # daughter chromosomes = # parent chromosomes Two identical daughter cells Occurs in all cells except reproductive cells Produces diploid cells Diploid cells = 2n Meiosis # daughter chromosomes = half the # parent chromosomes Four non-identical daughter cells Occurs only in reproductive cells Produces haploid cells Haploid cells = n

Review of Mitosis vs. Meiosis

Meiosis Details Gametes do not form equally in males and females In males, all 4 cells resulting from meiosis develop into sperm. In females, 1 cell gets most of the cytoplasm and becomes the egg.

Meiosis Details Chromosome mutations sometimes occur spontaneously Chromosome changes during meiosis can cause changes in the genetic information. Parts of chromosomes can be inverted, deleted, duplicated or moved to another spot. Chromosome mutations can occur because of mutagens Chromosome changes, sometimes leading to genetic disease or death, can be cause by mutagens such as radiation or chemicals.

Meiosis Details Failed separation of chromosomes in meiosis has serious consequences Failed separation means that a gamete may end up with no chromosome or too many of a chromosome. Zygotes that result from these gametes rarely survive, and if they do, they will have serious genetic disorders.

Karyotypes A diagram that shows homologous pairs of chromosomes identified and paired by size, centromere location, and banding patterns. Karyotypes are prepared by cutting and pasting chromosomes taken from body cells during mitosis.

Karyotypes Geneticists can determine when whole chromosome mutations have occurred. Syndrome – a particular disease or disorder with a specific group of symptoms that occur together.

What type of Karyotype?

What type of Karyotype?

Chromosome Error? Down Syndrome

Chromosome Error? Edwards Syndrome

Advantages and Disadvantages of Asexual Reproduction Lack of genetic diversity All population is genetically identical – this means all individuals in the population are equally vulnerable to change. Changes (weather, disease, etc) can cause entire populations to be wiped out. Some methods of asexual reproduction produce offspring in close proximity Offspring may compete for food and space Advantages Only one parent needed No need to find a mate (no energy required); therefore organism can start to reproduce as soon as it is ready and conditions suitable. Process occurs quickly Organism can produce many offspring It takes a short time for offspring to mature Offspring can reproduce soon after being produced Offspring genetically identical to the parent As long as environmental conditions stay same, organism lives with same success as parent

Advantages and Disadvantages of Sexual Reproduction Search for a mate Energy required to find a mate Search for a mate might expose an individual to predators, disease, or harsh environmental conditions. Fewer offspring tend to be produced Takes longer for a population to grow, even under suitable conditions. Takes offspring longer to reach maturity Therefore, takes longer before offspring can produce sex cells and reproduce themselves. Protection of offspring Substantial amount of time and energy to raise offspring until independent of protection of one or both parents. Advantages Genetic Diversity Offspring genetically different from parents, so they may survive new diseases or threats that appear in a population. Allows offspring a chance to survive Protection to embryo and offspring Organism tend provide more protection to embryo and take care of offspring after born.