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Case Study ? Mechelle and her peirced ears ! What’s a keloid ?. Chapter 18. Patterns of Chromosome Inheritance. Outline. Introduction Chromosomes and the Cell Cycle Mitosis Phases of Mitosis Meiosis Stages of Meiosis Crossing-Over Comparison of Meiosis and Mitosis
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Case Study? • Mechelle and her peirced ears ! • What’s a keloid? Chapter 18 Patterns of Chromosome Inheritance
Outline • Introduction • Chromosomes and the Cell Cycle • Mitosis • Phases of Mitosis • Meiosis • Stages of Meiosis • Crossing-Over • Comparison of Meiosis and Mitosis • Chromosome Inheritance
Introduction / Chromosomes and the Cell Cycle • 3 Factors determining the make-up of an individual: • Physical characteristics (“genes”). • Environment. • Spiritual condition. • Genetics is the study of heredity. . . . . • the study of how traits are passed from one generation to the next.
What is the function of cell division? • Growth > “mitosis” • Repair > “mitosis” • Reproduction > “meiosis” • Chromatin / chromosome • DNA ~ 100,000 genes ? “recipe” for you ! • Human Genome Projectestimated only ~ 33,000 genes !!! • Now say, 20-25,000 genes !!!!!!!!!
Each organism has a characteristic # of chromosomes (humans = 46 or 23 pairs) that can be identified by a karyotype. . . . . • an arrangement of all chromosomes within a cell by pairs in a fixed order. • In humans: • 22 pairs of non-sex chromosomes (autosomes) • 1 pair of sex chromosomes. . . . • XX – female, XY – male. • Thus, who determines the child’s sex?
Prior to cell division – • each chromosome is composed of 2 genetically identical parts termed chromatids (or sister chromatids) held together at a region called a centromere.
Cell Increase and Decrease • These represent opposing processes which by their very nature maintain balance (homeostasis) in the human organism. • Cell division increases the # of body cells (somatic cells). • Cell division = mitosis (division of the nucleus) and • Cytokinesis (division of the cytoplasm). • Decreasing the # of cells involves programmed cell death (apoptosis).
Obtaining Fetal Chromosomes • At times, the doctor and parents may want to view an unborn child’s chromosomes (karyotype) to see if there is a correct number or if a genetic disorder may be indicated. • Syndrome – a group of symptoms that always occur together. • Methods to obtain a sample: • Chorionic Villi Sampling (CVS) • Amniocentesis
The Cell Cycle • is an orderly set of stages that take place between the time a cell divides and the time the resulting cells also divide. • In order to understand the cell cycle, one must recall the structure of the cell. . . . . . . . • the cell membrane, cytoplasm (containing the organelles) and the nucleus. • Also, one must recall that when a cell is not dividing, the DNA (genetic material in the nucleus) and associated proteins are a tangled mass of thin treads called chromatin rather than the distinct “rod-like” chromosomes seen during cell division.
Cell Cycle • Cell cycle consists of interphase and mitosis: • Interphase (a period of growth and differentiation). • Mitosis (when the nucleus divides). • Cytokinesis (when the cytoplasm / cell divides). • Interphase is the interval of time between cell divisions, is the phase the cell is in the longest, and is not a time of rest but rather is when the cell is active carrying on its functions.
The Stages of Interphase • In mammalian cells it last about 20 hours (90% of the cell cycle). • Interphase is divided into 3 stages: • G1 stage – cell doubles its organelles, accumulates the materials needed for DNA synthesis. • S stage – DNA replication occurs, a copy is make of all the DNA in the cell. • G2 stage – cell synthesizes the proteins needed for cell division (ie. protein in microtubules).
The Mitotic Stage • Follows interphase. • It is called the M stage (for mitotic stage). • It includes mitosis and cytokinesis. • Mammalian cells require about 4 hours to complete the mitotic stage.
Mitosis (= duplication division) • Mitosis occurs in humans when tissues grow or when repair occurs, and produces 2 daughter cells with the same set (number) of chromosomes as the mother cell. • Before mitosis, the chromatin duplicates and shortens into chromosomes, each consisting of 2sister chromatids which are held together at the centromere. • Sister chromatids separate, and one of each kind of chromosome goes into each daughter cell.
Phases of Mitosis • Prophase. • Centrioles outside nucleus duplicate and move away from each other. • Spindle fibers appear. • Chromosomes become distinct, nuclear envelope fragments (membrane disappears), nucleolus disappears. • Metaphase. • Spindle fully-formed. • Chromosomes align single file at the equator. • Repel each other and appear “X-shaped”.
Stages of Mitosis • Anaphase. • Sister chromatids separate and “daughter chromosomes” move to the poles. • Spindle fibers shorten and pull chromosomes towards the poles. • Telophase. • When daughter chromosomes arrive at each pole. • Cytokinesis (via cleavage furrow) occurs. • Spindle disappears, nucleoli reappear, and the nuclear envelopes form. • Chromosomes become indistinct chromatin. • Have twoidenticaldaughter cells.
Control of the Cell Cycle • Is by internal and external chemical signals. • Growth factors are external signals received at the plasma membrane which can cause the cell to undergo the cell cycle. • Steps: reception, transduction, activate genes, response. • Cyclin is an internal signal that increases and decreases as the cell cycle continues. • It must be present for the cell to proceed from the G2 to the M stage, and from the G1to the S stage. • If DNA damage occurs, the protein p53 attempts to repair the DNA. • However, if DNA repair is not possible, p53 brings about cell death (apoptosis).
Apoptosis • is often defined as programmed cell death. • This is because the cell progresses through a usual series of events that bring about its destruction. . . . . . • the cell rounds up, looses contact with its neighbors the nucleus fragments the plasma membrane develops blisters (blebs) the cell fragments are engulfed by WBC’s or neighboring cells. • Apoptosis is actually facilitated by 2 sets of enzymes within the cell, called caspases, which include “initiators” and “executioners”.
Meiosis (= reduction division) • Meiosis requires two nuclear divisions and results in four daughter cells, each with half the number of parental chromosomes (haploid, n), one of each of the original 23 pair. • Humans have 23 pairs of homologous chromosomes. • During meiosis I: homologues pair and synapsis occurs allowing crossing-over. • Exchange of genetic material between nonsister chromatids of homologous pairs.
Next, the homologous chromosomes of each pair separate so that one chromosome from each pair will be in the daughter cell. • This reduces the number of chromosomes to half.
Meiosis • At the beginning of Meiosis II, the haploid (n) number of chromosomes per cell are dyads because each is still composed of two sister chromatids (attached at the centromere). • During Meiosis II, sister chromatids separate in each of the cells from Meiosis I. • Each of the resulting four daughter cells has the haploid number of chromosomes.
Stages of Meiosis • First Division. • Prophase I - Spindle appears; nuclear envelope fragments; homologues pair, synapse, crossing-over occurs. • Metaphase I - Tetrads line up at equator. • Anaphase I - Homologous chromosomes of each pair separate and move to opposite poles of the spindle, effectively dividing the chromosomes number in half (2n n). • Telophase I - Spindle disappears and the nuclear envelope reforms briefly. • Cytokinesis - Plasma membrane furrows and the cell waits momentarily during interkinesis.
Stages of Meiosis • Second Division. • Prophase II - Spindle reappears and nuclear envelope disassembles. • Metaphase II - Dyads line up at equator. • Anaphase II - Sister chromatids separate and move towards poles. • Telophase II - Spindle disappears and nuclear envelope reforms. • Cytokinesis - Plasma membrane furrows. • Four haploid (n) daughter cells produced.
The Importance of Meiosis • Due to meiosis, the number of chromosomes stay the same in each successive generation (Adam + Eve). • Also, the process assures that each new individual will have a slightly different genetic combination than either parent in 3 ways: • Crossing-over recombines genes located on homologous chromosomes derived from both parents. • Each gamete has a different combination of chromosomes. • Fertilization recombines chromosomes.
Mitosis Occurrence – throughout the body. Process – 1 cell division. Daughter cells – are diploid (2n) or identical to the mother cell. Results in – somatic cells (body cells) Meiosis Occurrence – only in the reproductive organs (testes or ovaries). Process - 2 cell divisions. Daughter cells - are haploid (n). Results in – gametes (germ cells, sex cells, sperm or ova) Mitosis Compared to Meiosis
Human Life Cycle • Mitosis ensures every cell has a complete number of chromosomes with every cell division. • Meiosis reduces the chromosome number by half in gametes (sex cells, germ cells). • Sperm and egg (gametes) are haploid (n). • Somatic cells are diploid (2n). • Mitosis – “cell division” • Meiosis – “reduction division”
Spermatogenesis and Oogenesis • Spermatogenesis occurs in the testes of males and produces haploid sperm. • Once started, continues to completion. • Oogenesis occurs in the ovaries of females, and produces haploid eggs. • Does not necessarily go to completion.
Considering the topics of the haploid nature of sex cells and related topics . . . . . What about the virgin birth of Jesus?
Chromosome Inheritance • Remember, humans have 22 pairs of autosomes, and one pair of sex chromosomes. • Abnormal chromosome number or structure often leads to a syndrome. . . . • Syndrome? Techniques to foreworn? • Amniocentesis and Chorionic Villi Sampling (CVS) can be used to obtain a genetic sample to produce a karyotype. • The visual display of chromosomes arranged by size, shape, and banding pattern.
