Chapter 2 Chromosomes and Cellular Reproduction
Sources of genetic variation
-Crossing over -Independent assortment -Random fertilization.
G₀ phase
-Sometimes a cell exits G₁ and enters this phase. -Here the cell is in a stable, nondividing state. -It can remain in this state indefinitely.
Checkpoints
-These are key transition points that regulate a cell's progression through the cell cycle. -If a cell doesn't meet all the requirements to move on to the next stage, then these will stop it.
Spermatogonia
-These are the cells in the testes that can enter meiosis I or divide mitotically to produce more of them. -They are diploid.
Spermatids
-These are the four haploid daughter cells that result when both secondary spermatocytes have finished meiosis II. -They eventually mature into sperm.
Secondary spermatocytes
-These are the two haploid daughter cells that result when the primary spermatocyte has finished meiosis I. -They go on to enter meiosis II.
Oogonia
-These diploid cells in the ovaries can either undergo repeated rounds of mitosis or enter meiosis I.
G₁/S checkpoint
-This checkpoint keeps the cell in the G₁ phase until all the proteins necessary for DNA replication are made. -After the cell passes this checkpoint, it is committed to divide.
Spindle-assembly checkpoint
-This checkpoint prevents the cell from continuing to anaphase unless all the spindle fibers are properly attached to the kinetochores.
G₂/M checkpoint
-This checkpoint stops the cell from progressing into the M phase until the DNA is replicated correctly and is undamaged.
Recombination
-This creates new combinations of alleles on a chromatid. -It is accomplished by crossing over.
Cytokinesis
-This is the division of the cytoplasm at the end of mitosis. -It is often simultaneous with telophase.
Separase
-This is the enzyme that breaks down cohesion so that the chromatids can separate.
Secondary oocyte
-This is the female haploid daughter cell that results from meiosis I. -It will then undergo meiosis II to produce a ovum and a second polar body. -The ovum can be fertilized by a sperm while the second polar body will disintegrate.
Ovum
-This is the female haploid gamete that is produced in meiosis II. -The secondary oocyte divides into this and a second polar body.
Telophase
-This is the fifth and final stage of mitosis. -It starts when the two sets of sister chromatids each arrive at their end of the cell. -Once there, the chromosomes decondense and a new nuclear envelope reforms around each set. -This stage is often simultaneous with cytokinesis.
Meiosis I
-This is the first division of meiosis. -It is referred to as the reduction division because the number of chromosomes per cell is cut in half.
G₁ (gap 1) phase
-This is the first part of interphase. -During it, the cell grows and makes the proteins necessary for DNA replication.
Prophase I
-This is the first stage of meiosis I. -During it, the following things occur: 1. The chromosomes condense 2. Homologous chromosomes synapse 3. Crossing over occurs 4. The nuclear envelope breaks down 5. The mitotic spindle forms.
Prophase II
-This is the first stage of meiosis II. -During it, the following things happen: 1. The chromosomes condense 2. The spindle fibers form 3. The nuclear envelope disintegrates
Prophase
-This is the first stage of mitosis. -During it, the following things happen: 1. The chromosomes condense 2. The mitotic spindle forms
Telophase I
-This is the fourth and final stage of meiosis I. -During this stage, the chromosomes arrive at the poles of the cell and the cytoplasm divides.
Telophase II
-This is the fourth and final stage of meiosis II. -Here, the chromatids arrive at each end of the cell, the spindle fibers break down, and the nuclear envelope reforms. -It occurs along with cytokinesis.
Anaphase
-This is the fourth stage of mitosis. -Here, the sister chromatids separate and get pulled to opposite ends of the cell by the spindle fibers.
Fertilization
-This is the joining of two haploid gametes (sperm and egg) in order to make a diploid zygote. -The fact that a random sperm and egg join together contributes to the genetic variation within populations.
G₂ (gap 2) phase
-This is the last phase of interphase. -During it the cell continues to prepare for cell division.
Cell cycle
-This is the life cycle of the cell. -It starts when a new cell is formed and concludes when it finishes dividing. -It consists of interphase and the M phase.
Primary oocyte
-This is the name for a oogonium that has entered meiosis I. -It will divide into a secondary oocyte and a first polar body. -The secondary oocyte will continue through meiosis II while the first polar body will disintegrate.
Bivalent or tetrad
-This is the name for a pair of homologous chromosomes that consists of a total of 4 chromatids. -They form through the synapsis of homologous chromosomes in prophase I of meiosis.
Primary spermatocyte
-This is the name for a spermatogonium that has entered meiosis I.
Tubulin
-This is the name for the molecule that makes up the spindle fibers. -It is broken down at both ends during anaphase in order to move the sister chromatids towards opposite poles of the cell.
Crossing over
-This is the name for the process by which a homologous pair exchanges genes between nonsister chromatids. -It generates genetic variation among the gametes.
Oogenesis
-This is the name for the production of gametes in female animals. -It occurs in the ovaries.
Spermatogenesis
-This is the name for the production of gametes in male animals. -It occurs in the testes throughout the male's life.
Synapsis
-This is the pairing up of homologous chromosomes during prophase I of meiosis. -Remember that each of the two chromosomes in the homologous pair consists of two sister chromatids because the DNA was replicated in the S stage of interphase.
Meiosis
-This is the process by which 4 genetically variable haploid gametes are made from a single diploid cell. -The end result is that the chromosome number in each of the 4 daughter cells is half that of the parent cell. -It consists of two divisions. -Germ cells in the reproductive organs divide in this way.
Mitosis
-This is the process by which somatic cells divide. -The result is that each daughter cell is genetically identical to the parent cell. -There is no reduction in the number of chromosomes.
Meiosis II
-This is the second division of meiosis. -It is sometimes referred to as the equational division because there is no reduction in chromosome number. -It stages are very similar to that of mitosis.
S phase
-This is the second phase of interphase. -During this phase, the DNA is replicated. -After this phase, each chromosome consists of two sister chromatids.
M (mitotic) phase
-This is the second phase of the cell cycle. -It consists of mitosis and cytokinesis.
Metaphase I
-This is the second stage of meiosis I. -Here, the homologous pairs line up at the metaphase plate. -The orientation of each homologous pair is independent of every other homologous pair. -The mitotic spindles then attach to each chromosome.
Metaphase II
-This is the second stage of meiosis II. -Here, the individual chromosomes, each composed of two sister chromatids, line up along the metaphase plate. -Note that there is no pairing of homologous chromosomes, because there aren't any. Each cell contains 23 chromosomes, each made up of two sister chromatids.
Prometaphase
-This is the second stage of mitosis. -During it, the following things occur: 1. The nuclear envelope breaks down 2. The spindle fibers attach to the kinetochores
Chiasma (plural: chiasmata)
-This is the site on the chromosomes where crossing over takes place.
Anaphase I
-This is the third stage of meiosis I. -During it, the two chromosomes (each with two chromatids) separate and get pulled by the mitotic spindle towards opposite ends of the cell.
Anaphase II
-This is the third stage of meiosis II. -During this stage of meiosis II, the sister chromatids separate and the spindle fibers move them towards opposite poles of the cell.
Metaphase
-This is the third stage of mitosis. -Here, the chromosomes align along the metaphase plate.
Interphase
-This part of the cell cycle consists of the G₁, S, and G₂ phases. -It precedes both mitosis and meiosis.
Cohesion
-This protein holds chromatids together in mitosis and meiosis. -During anaphase, the enzyme separase breaks this down along the entire length of the chromosome so that the chromatids can separate.
Independent assortment
-This refers to the random assortment and separation of homologous chromosomes in metaphase I and anaphase I. -How each pair aligns and separates (maternal on one side and paternal on the other or visa versa) is random and independent of the others. -Each daughter cell will get a random combination of maternal and paternal chromosomes.
Interkinesis
-This stage that occurs between meiosis I and meiosis II. -During it, the nuclear membrane reforms around the chromosomes at each pole, the spindle fibers break down, and the chromosomes decondense.
Sexual reproduction
-This type of reproduction involves combining the genes from two parents in order to form offspring that is genetically different. -It involves the processes of meiosis and fertilization.
Viruses
-are neither prokaryotic nor eukaryotic -consist of a protein coat that surrounds the genetic material -can only reproduce inside a host cell -are genetically similar to their host cells
Homologous pair
-corresponding pair of chromosomes in a diploid cell -one chromosome in the pair comes from the mother and the other is the corresponding chromosome from the father -humans have 23, which means they have a total of 46 chromosomes -before DNA replication, each consists of two corresponding chromatids -after DNA replication, each consists to 2 pairs of sister chromatids
Nuclear matrix
-internal scaffolding of the nucleus that holds things in place -consists of many protein fibers
Kinetochore
-multiprotein complex that forms around the centromere -spindle fibers attach to this during cell division
Chromatin
-name for the combination of DNA and histones -makes up chromosomes
Eukaryote
-organisms that are made up of one or more eukaryotic cells -can be unicellular or multicellular -includes animals, plants, and fungi
Prokaryote
-organisms that are unicellular, each consisting of a single prokaryotic cell -includes the eubacteria and archaea
Centromere
-point on the chromosome where sister chromatids attach -the kinetochore forms here
Binary fission
-process by which prokaryotic cells divide -DNA begins to replicate at two origins of replication that then move towards either end of the cell -once the DNA has finished replicating the cell divides
Histones
-proteins that combine with DNA so that it can fold into tightly packed chromosomes -only found in eukaryotes
Haploid
-refers to a cell or organism that has only one set of genetic information or chromosomes -an example is human gametes
Diploid
-refers to a cell or organism that has two sets of genetic information or chromosomes: one set from each parent
Chromosomes
-rod-shaped structures found in the nucleus -consist of DNA coiled around histones -can be made up of either one or two chromatids -each has a centromere, at least two telomeres, and several origins of replication
Origins of replication
-sites on a chromosome where DNA replication begins -there are several of them on each chromosome
Telomeres
-the ends or tips of the chromosome -protect the genetic material from degradation
Nucleus
-the structure in eukaryotic cells that encloses the genetic information -not found in prokaryotic cells
Cell division
-this is how a cell reproduces -it requires three things to happen: duplicate the genetic material, separate the copies, divide the cell
Sister chromatids
-two copies of a chromosome that are joined at the centromere -each chromatid consists of just one DNA molecule -both of them together constitute a single chromosome
