Chapter 13: Meiosis Flashcards
Gene
Hereditary units passed from parents to offspring that contain coded genetic information
Locus
A gene’s specific location along the length of a chromosome
Asexual reporduction
A single individual is the sole parent and passes copies of all its genes to its offspring without the fussion of gametes
The genomes of offspring are virtually identical of the parent’s genome
Sexual reporduction
Two parents give rise to offspring that have unique combinations of genes inherited from two parents
Genetic variation is an important consequence of sexual reporduction
Karotype
Image of all the metaphase chromosomes of a cell arranged in pairs starting with the longest chromosome
Homologous chromosomes
Also called homologs; are identical pair of two sister chromosomes that each carry genes controlling the same set of inherited characteristics
One chromsome contains genes inherited from one parent
Sex chromosomes
Two distinct chromosomes that are not identical homologs and determine the sex of an organism
XX chomosomes code for females and XY chromosomes code for males in humans
Autosomes
All other chromosomes other than sex chromosomes that compose an idividuals genome
Diploid cell
A cell with a two chromosome set has a diploid number of chromosomes; abbreviated as 2n
In humans the diploid number is 2n = 46 for the total number of chromosomes in somatic cells
Haploid cell
A gamete cell that only contains one set of chromomes; abbreviated n
In humans the haploid number is n = 23; the set of 23 consists of 22 autosomes plus a single sex chromosome
An unfefrtililzed egg always contains an X chromosome while a sperm cell may contain an X or a Y chromosome
Zygote
A fertilized egg that results from fertilization- the union of gametes culminating in the fusion of their nuclei
A zygote is thus diploid because it contains two haploid sets of chromosomes (2n) bearing genes from both the mother and the father
Nonsister chromatids
A pair of one maternal and one paternal chromatid in a pair of homologous chromosomes
Meiosis
Reduces the nuber of chromosome sets from diploid to haploid; each daughter cell will only have half as many chromosomes as its parent
Like mitosis, meiosis is preceeded by the duplication of chromosomes during interphase
No prometaphase in meiosis
This single duplication is followed by two consecutive cell divisions called meiosis I and meiosis II
- Meiosis I separates homologous chromsomes
- Meiosis II separates sister chromatids
Homologs appear alike however may have different versions of genes called alleles
Prophase I
After interphase the sister chromatids are held together by proteins called cohesins
In early prophase I each chromosome pairs with its homolog and crossing over occurs in which nonsister chromatids are broken at precisely corresponding positions
- X-shaped regions called chiasmata are sites of crossover
Metaphase I
In metaphase I, pairs of homologs line up at the metaphase plate, with one chromosome facing each pole
In humans 23 pairs of homologous chromosomes line up along the metaphase plate instead of 46
Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad- four homologous chromsomes
Anaphase I
In anaphase I pairs of homologous chromosomes separate
One chromosome of each pair moves toward opposite poles, guided by the spindle apparatus
Sister chromatids remain attached at the centromere and move as one unit toward the pole
In humans 23 sister chromatids move toward each side of the cell
Telophase I and Cytokinesis
In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids
Cytokinesis usually occurs simultaneously, forming two haploid daughter cells
No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated
Prophase II
Metaphase II
In metaphase II the sister chromatids are arranged at the metaphase plate
In humans 23 sister chromatids line up along the metaphase plate
Due to crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical
The kinetochores of sister chromatids attach to microtubules extending from opposite poles
Anaphase II
In anaphase II the sister chromatids separate
The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles
In humans 23 individual chromosomes move towards each
Telophase II and Cytokinesis
In telophase II the chromosomes arrive at opposite poles
Nuclei re-form and the chromosomes begin decondensing
Cytokinesis separates the cytoplasm
Marks the end of meiosis resulting in four daughter cells, each with a haploid set of unreplicated chromosomes
Each daughter cell is genetically distinct from the others and from the parent cell
Crossing over process
Occurs during prophase I in meoisis
- The two members of a homologous pair associate loosely along their lengths
- The DNA of one paternal and one maternal non-sister chromatid align so that their genes correspond percisely with one another
- A zipper-like complex called the synaptonemal complex forms that holds homologous pairs tightly together
- The DNA of the non-sister chromatids are broken at percisely corresponding points
- During this association called synapsis the DNA breaks are re-joined so that the two non-sister chromatids each now contain DNA from the other
- After crossing over occurs the synaptonemal complex disassembles and the homologs move away from each other slightly forming a visible chiasmata where the crossing over occured
- The homologs remain attached because sister chromatid cohesion still holds them together although some DNA is no longer attached to its original chromosome
Unique differences between meiosis and mitosis
- Synapsis and crossing over
- In metaphase I homologous pairs line up at the metaphase plate rather than individual chromosomes
- In anaphase 1 homologous pairs are separated while the sister chromatids remain attached; sister chromatid cohesion is released in anaphase II
Origins of genetic variation among offspring
- Independent assortment of chromosomes
- Crossing over
- Random fertilization
Independent assortment of chromosomes
Each pair of chromosomes sorts maternal and paternal homologs into daughter cells independently of the other pairs
Each daughter cell thus represents one possible outcome among all possible combinations of maternal and paternal chromsomes
The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number
In humans n = 23 so there are about 8.4 million (223) possible combinations of chromosomes
Crossing over outcome
Crossing over produces recombinant chromosomes which combine DNA inherited from each parent and results in novel chromosomes
Contributes to genetic variation by combining DNA from two parents into a single chromosome
In humans an average of one to three crossover events occurs per chromosome
Random fertilization
Random fertilization adds to genetic variation because any sperm can fuse with any ovum- unfertilized egg
In humans the fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations
