BY 101 Study Guide

Chapter 9 – Cell Division

Cell division is an elegant process that enables organisms to grow and reproduce. Through a sequence of steps, the replicated genetic material (chromosomes) in a parent cell is equally distributed to two daughter cells. While there are some subtle differences, mitosis is remarkably similar across organisms.

Before a dividing cell enters mitosis, it undergoes a period of growth called interphase. Interphase is the "holding" stage or the stage between two successive cell divisions. In this stage, the cell replicates its genetic material and organelles in preparation for division.

The cell cycle, or cell-division cycle, is the series of events that take place in a cell leading to its division and duplication (replication).

The cell cycle consists of four distinct phases: G1 phase, S phase (synthesis), G2 phase (collectively known as interphase) and M phase (mitosis). M phase is itself composed of two tightly coupled processes: mitosis, in which the cell's chromosomes are divided between the two daughter cells, and cytokinesis, in which the cell's cytoplasm divides forming distinct cells. Activation of each phase is dependent on the proper progression and completion of the previous one. Cells that have temporarily or reversibly stopped dividing are said to have entered a state of quiescence called G0 phase.

Mitosis is composed of several stages: * Prophase * Metaphase * Anaphase * Telophase
Prophase
In prophase, the chromatin (DNA) condenses into discrete chromosomes. The nuclear envelope breaks down and spindles form at opposite "poles" of the cell.
Metaphase
In metaphase, the chromosomes are aligned at the metaphase plate (a plane that is equally distant from the two spindle poles).
Anaphase
In anaphase, the paired chromosomes (sister chromatids) move to opposite ends of the cell.
Telophase
In this last stage, the chromosomes are cordoned off in distinct new nuclei in the emerging daughter cells. Cytokinesis is also occurring at this time.

At the end of mitosis, two distinct cells with identical genetic material are produced.
Chapter 10 – Meiosis
Meiosis is a process of reductional division in which the number of chromosomes per cell is cut in half (i.e., from DIPLOID to HAPLOID). In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores.
During meiosis, the genome of a diploid germ cell, which is composed of long segments of DNA packaged into chromosomes, undergoes DNA replication followed by two rounds of division, resulting in four haploid cells.
In humans, the diploid number is 46 (or 23 pairs). This number is reduced to 23 following meiosis.
PHASES OF MEIOSIS (see Fig. 10.5)
Meiosis I
Late Interphase:
Chromosomes replicate into two identical sets right before meiosis begins.
Prophase I:
Nuclear membrane dissolves.
Chromosomes have replicated and identical pairs are attached forming "Sister Chromatids". Similar chromosomes pair with one another forming homologous pairs. Crossing over can occur at this time.
Metaphase I:
Sister chromatids/ homologous pairs line up at the equator or middle of the cell.
Anaphase I:
The homologous pairs of chromosomes separate and one chromosome from each pair is pulled to an opposite pole. Each chromosome is still composed of two chromatids joined by a centromere.
Telophase I:
Cell divides into two daughter cells, each of which begins another division=Meiosis II
The sister chromatids are still joined.
Meiosis II
Prophase II:
Each cell contains one member of each homologous chromosome pair. The chromosomes are not copied again.
Metaphase II:
The sister chromatids line up at the center of the cell.
Anaphase II:
Sister chromatids are separated and move to opposite sides of the cell.
Telophase II:
Nuclei form around each group of chromosomes in each daughter cell. Cytokinesis occurs.
The two nuclear divisions in meiosis result in four daughter cells forming from an original parent cell, each with half the chromosomes of the parent cell.
Chapter 11 – Heredity
Mendel is considered the father of modern genetics. He studies pea plant inheritance. In his monohybrid studies he observed dominant and recessive traits. Dominant traits are observed more often than recessive traits (i.e., purple versus white flowers on pea plants).
Mendel’s first law states that alleles are in pairs (i.e., we have homologous chromosomes) and they segregate during meiosis.
Mendel also observed if traits were inherited together….such as dominant traits like flower color and flower height….in dihybrid experiments.
From these studies he came up with a second law. The second law states that genes for different traits (i.e., different homologous chromosomes) sort independently of one another during meiosis.
Genotype – the actual genetic makeup of an individual (i.e., AA, Aa, or aa).
Phenotype – the result of a given genotype (i.e., AA and Aa genotypes express “A”)
Codominant traits – alleles that both act dominant and are both expressed (i.e., persons with A and B blood type genes are type AB blood).
Incomplete dominance – alleles that tend to be expressed as a “blended” phenotype (i.e., red and white flowers crossed to give pink flowered offspring).
Pleiotropy – when one gene gives rise to multiple characters (i.e., if one genes coded for hair color, eye color, and skin color).
Epistasis – when one gene masks the expression of another gene. (e.g., ee gene for mammalian coat color masks the genes for coat color (“B” genes).
Remember, just because you inherited a gene (or gene pair) doesn’t mean you will express, or use, that gene. The environment inside and outside a cell, or organism, dictates how many genes are expressed. (e.g., coat color in mammals can be effected by environmental temperature).
Chapter 12 – Human Inheritance
Humans have 23 pairs of homologous chromosomes.
The first 22 pairs of chromosomes are considered autosomes and contain many genes.
The 23rd pair is considered the sex, or gender, chromosomes. They are called this because they contain many genes on them that code for either male or female traits. An individual with two “X” chromosomes for that pair is a female and an individual with an “X” and a “Y” chromosome for that pair is a male.
X-linked traits are those carried on an “X” chromosome. Males are more likely to express X-linked traits since they only get one “X” chromosome. Examples of X-linked traits include hemophilia and color blindness.
Autosomal recessive traits are not observed as often as autosomal dominant traits.
Autosomal recessive traits can only occur if two heterozygous individuals decide to have kids (i.e., Aa x Aa). This mating has a ¼ chance of producing aa individuals. An example of an autosomal recessive traits includes galactosemia.
Autosomal dominant traits can be passed on if only one of a mating pair has the dominant gene (i.e., Aa x aa). In this cross, ½ of the offspring have a chance of getting the “A” allele. Examples of autosomal dominant traits include achondroplasia and Huntington’s disease.
Abnormal events can cause changes in chromosome structure and/or number.
Duplications, deletions, inversions and translocations can all result in abnormal chromosomes.
Diseases associated with these events include Hunington’s disease (the result of a duplication event), Cri-du-chat syndrome (the result of a deletion event), infertility (the result of an inversion event), and CML (chronic myelogenous leukemia) (the result of a translocation event).
Nondisjuction, occurring during meiosis, can result in sperm or eggs that have abnormal chromosomal content. Cells end up aneuploid (i.e., having either too many or too few chromosomes). Trisomy and monosomy are examples of aneuploidy. A known trisomy of major concern to humans is trisomy 21 (Down syndrome).
Abnormal sex chromosome combinations include XXY (Klinefelter’s syndrome), XYY, and XO (Turner’s syndrome). Many of these individuals are not fertile and cannot reproduce.
Social counseling often involves the use of pedigrees (i.e., charts showing the family history of a trait or condition) to predict the likelihood of a mating couple having children with a certain trait (often a disease condition).
Karyotypes and other screening methods (i.e., prenatal diagnosis such as amniotic fluid screening) can often be used to screen a human fetus for a genetic abnormality.