Published on March 8, 2014
BY L.N MASIA LIFE SCIENCES
In humans, somatic cells (body cells) have: 23 pairs of homologous chromosomes and one member of each pair from each parent. The human sex chromosomes (Gonosomes) X and Y differ in size and genetic composition. The other 22 pairs of chromosomes are autosomes with the same size and genetic composition. .
Homologous chromosomes are matched in: length, Centromere centromere position, and gene locations (locus). A locus (plural, loci) is the position of a gene. Different versions (alleles) of a gene may be found at the same locus on maternal and paternal chromosomes.
Homologous chromosome pair Centromere
Humans and most animals and plants have diploid body cells. That means they have two sets of chromosomes (homologous chromosome pair) one from each parent. Diploid is written 2n. It refers to the total number of chromosomes a cell can have.
Meiosis is a process that converts diploid nuclei to haploid nuclei. Diploid cells have 2 sets of chromosomes. Haploid cells have 1 set of chromosomes. Meiosis occurs in the sex organs, producing gametes—sperm and eggs. Fertilization is the fusion of a sperm and egg cell. The zygote has a diploid chromosome
Haploid gametes (n 23) A life cycle n Egg cell n Sperm cell Meiosis Ovary Fertilization Testis Diploid zygote (2n 46) 2n Key Multicellular diploid adults (2n 46) Mitosis Haploid stage (n) Diploid stage (2n)
All sexual life cycles include an alternation between a diploid stage and a haploid stage. Why is meiosis so important? It produces haploid gametes which prevents the chromosome number from doubling in every generation. Produce gametes for fertilization.
3. MEIOSIS Meiosis is a type of cell division that produces haploid gametes from diploid cells. Two haploid gametes combine in fertilization to restore the diploid state in the zygote. 3
SUMMERY OF THE MEIOSIS PROCESS 3
MEIOSIS I consisting of 5 phases: Interphase I, Prophase I, Metaphase I, Anaphase I, Telophase I. MEIOSIS II consisting of 4 phases Prophase II, Metaphase II, Anaphase II, Telophase II.
Cell build up energy DNA Replication (to make duplicated chromosomes Cell doesn’t change structurally.
Events occurring in the nucleus: Chromosomes coil and become individual chromo-somes, nucleolus and nuclear envelope disappear. Homologous chromosomes come together as pairs by synapsis forming a tetrad (Each pair, with four chromatids) Non-sister chromatids exchange genetic materi through the process of crossing over to ensure genetic variation. Centrioli move to opposite poles with spindle fibers between them.
Genetic recombination is the production of new combinations of genes due to crossing over. Crossing over is an exchange of genesbetween separate (non-sister) chromatids on homologous chromosomes. Non-sister chromatids join at a chiasma (plural, chiasmata), the site of attachment. Genetic material are exchanged between maternal and paternal (nonsister)
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Centrioli has reached the poles. Homologous pairs align at the cell equator. The two chromosomes attach to one spindle fiber by means of the kinetochore of the centromere. .
Spindle fibers contract. Duplicated chromosomes move to opposite poles. .
Duplicated chromosomes have reached the poles. A nuclear envelope and nucleolus re-forms around chromosomes. Each nucleus now has the haploid number of chromosomes. Cell invaginates forming a cleavage furrow, which extends to for 2 separate haploid cells.
Follows meiosis I without chromosome duplication. Each of the two haploid products enters meiosis II.
Chromosomes coil and become compact (if uncoiled after telophase I). Nuclear envelope and nucleolus, if re-formed, dissappears again. Centrioli move to opposite poles, forming spindle fibers between them.
Individual duplicated chromosomes align on the equator. One chromosome per spindle fiber attached by means of kinetochore of centromere. Centrioli has reached the poles.
Spindle fibers contract. Duplicated chromosomes split in half (centromere dividing in 2) Daughter chromosomes move to opposite poles.
Daughter chromosomes has reached the poles. Two cells invaginate and form 4 daughter haploid cells (gametes) They uncoil and form chromatin. Nuclear envelope and nucleolus for around chromatin again. Centrioli for centrosome.
Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Haploid daughter cells forming
Mitosis and meiosis both begin with diploid parent cells that have chromosomes duplicated during the previous interphase. However the end products differ. Mitosis produces two genetically identical diploid somatic daughter cells. Meiosis produces four genetically unique haploid gametes.
Independent orientation at metaphase I Random fertilization. Crossing over of genes during prophase I © 2012 Pearson Education, Inc.
6. KARYOTYPE • A karyotype is an ordered display of magnified images of an individual’s chromosomes arranged in pairs. • Karyotypes allow for the observation of : homologous chromosome pairs, chromosome number, and chromosome structure. © 2012 Pearson Education, Inc.
Centromere Sister chromatids Pair of homologous chromosomes Sex chromosomes
An extra copy of chromosome 21 causes Down syndrome or also known as TRISOMY 21. A. Trisomy 21 involves the inheritance of three copies of chromosome 21 and is the most common human chromosome abnormality.
Trisomy 21 produces a characteristic set of symptoms, which include: mental retardation, characteristic facial features, short stature, heart defects, susceptibility to respiratory infections, leukemia, and Alzheimer’s disease, and shortened life span. The incidence increases with the age of the mother.
Nondisjunction is the failure of chromosomes or chromatids to separate normally during meiosis. This can happen during: meiosis I, if both members of a homologous pair go to one pole or meiosis II if both sister chromatids go to one pole. Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes.
MEIOSIS I Nondisjunction MEIOSIS II Normal meiosis II Gametes Number of chromosomes n1 n1 n1 Abnormal gametes n1
MEIOSIS I Normal meiosis I MEIOSIS II Nondisjunction n1 n1 Abnormal gametes n n Normal gametes
Sex chromosome abnormalities tend to be less severe, perhaps because of the small size of the Y chromosome or X-chromosome inactivation.
In general, a single Y chromosome is enough to produce “maleness,” even in combination with several X chromosomes, and the absence of a Y chromosome yields “femaleness.”
Chromosome breakage can lead to rearrangements that can produce: genetic disorders or, if changes occur in somatic cells, cancer.
a deletion, the loss of a chromosome segment, a duplication, the repeat of a chromosome segment, an inversion, the reversal of a chromosome segment, or a translocation, the attachment of a segment to a nonhomologous chromosome that can be reciprocal.
Honors Biology Chapters 10 - 13 Law of Segregation 2 alleles for a character segregate when gametes are formed Behavior of chromosomes during meiosis
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