bio lecture 14 exam 2 flashcards (copy)
Diploid and Haploid Cells
Diploid Cells:
Definition: Cells that have two sets of chromosomes, one inherited from each parent (2n). Most eukaryotic organisms are diploid at least at one point in their life cycle, typically the somatic cells.
Example: Humans have 46 chromosomes (or 23 pairs), where n=23, so 2n=46. Mammals and vertebrates, including humans, are diploid organisms.
Haploid Cells (Gametes):
Definition: Cells that have one set of chromosomes (n), such as sperm and egg cells.
Importance: During sexual reproduction, the fusion of two haploid gametes (sperm and egg) restores the diploid state (zygote), preventing the formation of tetraploid organisms or other aneuploidies during fertilization.
Meiosis Overview
Definition of Meiosis:
A specialized type of cell division process that creates four haploid gametes (sex cells) from one diploid germline cell.
Also known as "reduction division" because it reduces the chromosome number by half from diploid (2n) to haploid (n).
Purpose of Meiosis:
To generate genetic diversity in offspring through mechanisms like crossing over and independent assortment, leading to unique haploid cells.
Ensures the correct chromosome number is maintained across generations; during fertilization, the haploid gametes combine to form a diploid embryo with the species-specific chromosome count.
Steps in Meiosis:
Meiosis involves two consecutive rounds of cell division without an intervening DNA replication phase:
Meiosis I (reductional division)
Meiosis II (equational division)
Each division has stages akin to mitosis (prophase, metaphase, anaphase, telophase), but the outcomes differ significantly due to unique events in Meiosis I.
Comparison of Mitosis vs Meiosis
Mitosis:
Produces two genetically identical diploid somatic cells (2n) from a single diploid cell. Its primary functions include growth, tissue repair, and asexual reproduction.
Involves one division cycle (prophase, metaphase, anaphase, telophase), where sister chromatids separate.
Meiosis:
Produces four genetically distinct haploid cells (n) (gametes or spores) from a single diploid cell. It is essential for sexual reproduction.
Involves two successive divisions:
Meiosis I: Homologous chromosomes separate after mixing genetic material through crossing over, resulting in two haploid cells with duplicated chromosomes (each chromosome still has two sister chromatids) and unique combinations.
Meiosis II: Sister chromatids separate in each of the two cells formed in Meiosis I, resembling mitosis but occurring in haploid cells, to finalize the formation of four haploid cells.
Stages of Meiosis
Meiosis I
Prophase I:
Chromosomes condense and thicken. Homologous chromosomes (one from each parent) pair up precisely, forming bivalents in a process called synapsis. Each bivalent consists of four chromatids (a tetrad).
Crossing Over: An exchange of genetic material between non-sister chromatids of homologous chromosomes occurs at chiasmata, leading to new combinations of alleles (genetic recombination) and significant genetic variability.
Metaphase I:
Homologous pairs (tetrads) align randomly along the metaphase plate in the center of the cell. The orientation of each homologous pair is independent of other pairs, contributing to independent assortment.
Anaphase I:
Homologous chromosomes are pulled to opposite poles of the cell by spindle fibers. Crucially, sister chromatids remain attached at their centromeres and move as a unit toward the same pole.
Telophase I:
Chromosomes arrive at the poles, and nuclear envelopes may reform around the two sets of chromosomes. Each pole now has a haploid set of chromosomes, but each chromosome still consists of two sister chromatids. Cytokinesis usually follows, resulting in two new haploid cells.
Meiosis II
Prophase II:
A brief prophase where chromosomes condense again, and a new spindle apparatus forms in each of the two haploid cells. No further DNA replication or homologous pairing occurs.
Metaphase II:
Chromosomes line up individually along the metaphase plate in a single file, similar to metaphase in mitosis.
Anaphase II:
Sister chromatids finally separate at the centromeres and move to opposite poles of the cell, becoming individual chromosomes.
Telophase II:
Chromosomes decondense at the poles, nuclear envelopes reform, and cytokinesis occurs. This results in the formation of four genetically distinct haploid daughter cells, each containing a single set of unduplicated chromosomes.
Genetic Diversity in Meiosis
Importance:
Meiosis is critical for generating diverse offspring, which enhances the adaptability and increases the chances of survival of a species against changing environmental challenges.
Mechanisms for generating diversity:
Crossing over: Creates recombinant chromatids with new combinations of alleles on the same chromosome.
Independent assortment: The random alignment of homologous chromosome pairs during Metaphase I means that each gamete receives a unique combination of paternal and maternal chromosomes. For humans (23 pairs), this allows for 2^{23} possible combinations.
Random fertilization: The fusion of any one of millions of unique sperm with any one of millions of unique eggs further amplifies genetic variation among offspring.
Nondisjunction
Definition:
A chromosomal mishap that occurs during cell division when homologous chromosomes (in Meiosis I) or sister chromatids (in Meiosis II or mitosis) fail to separate properly. This leads to an abnormal number of chromosomes (aneuploidy) in the resulting gametes or daughter cells.
Consequences:
Trisomy: The presence of an extra chromosome (2n+1), where an individual has three copies of a particular chromosome instead of the normal two. A well-known example is Down syndrome, caused by trisomy of chromosome 21.
Monosomy: The absence of one chromosome (2n-1), where an individual has only one copy of a particular chromosome. This condition is often lethal during embryonic development, but one notable exception is Turner syndrome (XO), involving the absence of one X chromosome.
Male vs Female Gamete Formation
Spermatogenesis
Definition: The continuous process of producing sperm cells (male gametes) in the testes of males through meiosis. It begins at puberty and continues throughout life.
Process: One primary spermatocyte (diploid) undergoes Meiosis I to form two secondary spermatocytes (haploid). Each secondary spermatocyte then undergoes Meiosis II to produce two spermatids, which mature into four functional, motile sperm cells.
Oogenesis
Definition: The process of producing egg cells (female gametes) in the ovaries of females through meiosis. This process is discontinuous and begins before birth, pauses, and then completes after puberty upon fertilization.
Process: One primary oocyte (diploid) undergoes Meiosis I to form one large secondary oocyte (haploid) and a small, non-functional first polar body (which may or may not complete Meiosis II). The secondary oocyte then arrests in Metaphase II and is released during ovulation. If fertilized, it completes Meiosis II, producing one large functional ovum and a small, non-functional second polar body. The polar bodies serve mainly to provide energy reserves and discard excess chromosomes, ensuring the ovum retains most of the cytoplasm and organelles.
Chromosome Count in Humans
Normal human diploid (2n) cells possess a total of 46 chromosomes, organized as:
22 pairs of autosomes (non-sex chromosomes), common to both males and females.
1 pair of sex chromosomes: which determine an individual's biological sex.
Females: XX (homologous pair)
Males: XY (non-homologous pair)
Sex Chromosome Disorders
These disorders are typically a result of nondisjunction of sex chromosomes during meiosis.
Turner Syndrome (XO):
Affects females who have only one X chromosome. Incidence is about 1 in 2,500 live female births.
Symptoms: Short stature, broader chest with widely spaced nipples, underdeveloped reproductive organs (streak gonads), leading to infertility, lack of menstruation (amenorrhea) and breast development, and potential heart defects or learning difficulties.
Klinefelter Syndrome (XXY):
Affects males who have an extra X chromosome. Incidence is about 1 in 500 to 1,000 live male births.
Symptoms: Underdeveloped testes, reduced fertility, limited facial and body hair distribution, and additional physical features such as tall stature, gynecomastia (breast development), and potential learning disabilities or speech problems. Often diagnosed at puberty when secondary sexual characteristics fail to develop normally.
Jacob's Syndrome (XYY):
Affects males who have an extra Y chromosome. Incidence is about 1 in 1,000 live male births.
Symptoms: Generally taller than average, may experience more severe acne problems during adolescence, and potential speech and reading difficulties. Most individuals have normal fertility and appearance; symptoms can be subtle and many go undiagnosed.
Triple X Syndrome (XXX):
Affects females who have an extra X chromosome. Incidence is about 1 in 1,000 live female births.
Symptoms: Usually taller than average, may have mild learning disabilities or developmental delays, and a slightly higher risk of psychiatric conditions. Physical characteristics are often subtle, and most XXX females have normal fertility.
Ethical and Practical Implications
Genetic studies in meiosis, including karyotyping and genomic sequencing, can help in diagnosing and understanding the etiology of genetic disorders, paving the way for new treatments, interventions, and family planning through genetic counseling.
By understanding the principles of genetic variation generated during meiosis, we can appreciate biodiversity, its importance in species adaptation and survival, and apply this knowledge in fields like selective breeding and conservation biology.