BISC105 Study Guide - Final Guide: Sister Chromatids, Chromosome Abnormality, Down Syndrome
11 Oct 2018
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Chromosomes
-Chromosomes are NOT DETECTABLE in non-dividing cells or nucleus (READILY
DETECTABLE in dividing cells
-Somatic cells have 46 chromosomes (23 pairs, Diploid/2N, one member of each pair donated
by each parent) Sex cells have 23 chromosomes (23 unpaired, Haploid/2N)
-SPERM (23 Chromosomes)+EGG (23 Chromosomes)=ZYGOTE (46 Chromosomes)
-Body cells have 44 autosomes (2 sex chromosomes), Sex cell have 22 autosomes (1 sex
chromsome)
-Autosome: Non-sex chromosome
1. The cell is the basic unit of our bodies and all living organisms.
2. There are basically two types of cells:
• Prokaryotes- simple, single circular chromosome
• Eukaryotes- complex, more DNA, many chromosomes
3. Eukaryotic cells have a well-defined nucleus which is the primary site of genetic
information, the genome.
4. The genetic information is found in chromatin which is a
filamentous material.
5. Two types of chromatin exist:
• Euchromatin– lighter staining, loosely coiled and genetically active
• Heterochromatin – darkly stained, tightly coiled and genetically inactive
Karyotype
•XX FEMALE
•XY MALE
•arranged in decreasing size (ex. 1 is largest, 22 is smallest)
-Chromosome replication BEFORE nuclear division
-Before replication: Each chromosome is a single rod (23 pairs) (2X DNA content)
-After replication: There are two sister chromatids in each of the 46 chromosomes (4X DNA
content)
-Each replicated chromosome has 2 DNA molecules, one in each of the sister chromatids
-Following replication, Nucleus can undergo cell division
CELL CYCLE
-Events related to the growth and division of a cell or the life history of a cell (3 Phases:
Interphase, Mitosis and Cytokinesis) ENTIRE PROCESS 25 HOURS
-INTERPHASE: (G1: interview of cell growth BEFORE replication (9.45 hours), S: longest
phase, replication takes place (10.5 hours), G2: following replication, cells prepare to divide,
proteins are made (2.3)
-MITOSIS (PMAT): Chromosomes are highly condensed and visible—>prophase (longest),
metaphase, anaphase, and telophase
-CYTOKINESIS: Cytoplasm divides
-Cells can leave the cell cycle and enter inactive stage of G0
PHASES OF MITOSIS
-PROPHASE: chromosomes are long and thin, centrioles divide and move to opposite poles,
nuclear envelope and nucleolus disappear
-METAPHASE: Chromosomes are shorter, highly nonsense, align across the equator of the
spindle fibers at the centromere
-ANAPHASE:Centromeres divide and the 2 chromatids of each chromosome separate, each
becomes a chromosome (92 chromosomes), move toward opposite poles
-TELOPHASE: 2 nuclei formed
-NUCLEAR DIVISION WHERE EACH REPLICATED CHROMOSOME SPLITS. DIPLOID
CELL—>2 IDENTICAL DIPLOID CELLS (maintains uniformity and stability,
quantitative&qualitative distribution)
Rates of Cell Division
-Rate regulated by genes (some have no control: diseases)
•Progeria: divisions halted and short of max #, premature aging (7-8 appears 70-80)
•Werner: 15-20 years old appears 45-50
•IN BOTH CONDITIONS, CELLS SWITCHED FROM GROWTH—>MAINTENCE MODE
-Different rates at different developmental stages (max # of cells under genetic control)
-Cell division important in tissue repair
-NON CYCLING CELLS: nervous system, brain and spinal cord in G0
-Cell division outside the body: in the lab in a flask, shows regulation and control
(HAYFLICK LIMIT: max # of cell divisions that can occur in a flask, 50 cell divisions)
-Uncontrolled cell division: Cancer
-CELLS ARE PROGRAMMED TO HAVE A CERTAIN NUMBER OF DIVISIONS
Gametes
-MALES: Spermatogenesis: making of sperm, occurs in testes
•begins at puberty, 48 days (16 meiosis I, 16 meiosis II, 15 spermatid—>mature sperm)
•sperm contain 22 autosomes and either and X or Y chromosome
•Seminiferous Tubules: contain many spermatocytes, large #s of sperm always in production,
millions of sperm over a lifetime
•Primary spermatocyte begins meiosis, 2 daughter cells (secondary spermatocytes, 4
haploid gametes (spermatids—>undergo differentiation to become sperm) (50% become X,
50% become Y)
-FEMALES: OOGENESIS
•Meiosis I
•starts during embryonic development, completed at ovulation
•starts prenatally, stops and continues during puberty
•Primary oocyte (frozen in dictotyne until puberty)—>2 cells of unequal size, the larger one
becomes secondary oocyte (large amount of cytoplasm) and other is the first polar body
(Non functional cells)
•Ovulates secondary oocyte each month
•Meiosis II
•only completed when fertilized
•First polar body divides—> 4 products, only
one becomes functional (ovum), and others
degenerate
-Older women’s eggs have harder time maintaining
normal chromosome number
-Continuous production of sperm over lifetime, eggs
have restricted production period and only fertilized
eggs complete meiosis II
Study of Human Chromosome
-CYTOGENETICS
-Usually prepared from blood cells
•Current Methods
•blood culture (white blood cells divide-in G0,
must be stimulated w/ mitogen, freezes cells at
metaphase)
•Culture of skin and organ fragments
•Culture of amniotic cells—in fluid (amniotic)
•Chorionic villi—surrounding the embryo
-Creates Karyotype
-22 Homologous pairs in males and females
(autosomes). X’s homologous, X’s and Y’s are not
-Because chromosomes exist as pairs, so do the
genes they carry
•ALLELES: alternate forms of the same gene,
carry same trait but are not identical
Meiosis
-produces 4 haploid cells containing only one copy of each chromosome
-MEOSIS I: REDUCTION DIVISION
•Same PMAT as mitosis
•Crossing over
-MEIOSIS II: EQUATIONAL DIVISION
•Metaphase II: unpaired chromosomes align at cell’s middle
•Anaphase II: centromeres divide—>chromosomes move to opposite poles
-INTERKENESIS: No DNA replication between nuclear divisions
-DAUGHTER CELLS IN MITOSIS NOT IDENTICAL
-SUMMARY OF CHROMOSOME BEHAVIOR IN MEIOSIS:
•members of chromosome pairs, each chromosome pairs with its homologue, paired
homologous chromosomes separate in meiosis I, sister chromatids separate and become
individual chromosomes during meiosis II
•Crossing over: Prophase I, chromatids touch each other and swap segments. Takes place
between non-sister chromosomes. Physical exchange of chromosome segments and genes
they carry
•New combinations of alleles
•Random Assortment: Meiosis 1, when chromosomes move apart to different daughter cells,
produces different combinations (ex. all father, all mother, mixture)
-IMPORTANT EVENTS OF MEIOSIS
1. REDUCTION OF CN FROM 46—>23 CHROMOSOMES
2. CROSSING OVER
3. RANDOM OR INDEPENDENT ASSORTMENT OF NON-HOMOLOGOUS
CHROMOSOMES
-SEX CELLS DO HAVE AUTOSOMES
Document Summary
Chromosomes are not detectable in non-dividing cells or nucleus (readily. Somatic cells have 46 chromosomes (23 pairs, diploid/2n, one member of each pair donated by each parent) sex cells have 23 chromosomes (23 unpaired, haploid/2n) Body cells have 44 autosomes (2 sex chromosomes), sex cell have 22 autosomes (1 sex chromsome) The cell is the basic unit of our bodies and all living organisms. Euchromatin lighter staining, loosely coiled and genetically active. Heterochromatin darkly stained, tightly coiled and genetically inactive. Karyotype: xx female, xy male, arranged in decreasing size (ex. Before replication: each chromosome is a single rod (23 pairs) (2x dna content) After replication: there are two sister chromatids in each of the 46 chromosomes (4x dna content) Each replicated chromosome has 2 dna molecules, one in each of the sister chromatids. Events related to the growth and division of a cell or the life history of a cell (3 phases: Interphase, mitosis and cytokinesis) entire process 25 hours.
