Meiosis and Gametogenisis Flashcards
What is Meiosis?
Meiosis is the process by which gametes are produced(gametogenesis)
What are homologous chromosomes
Chromosome pairs, similar in structure, that contain the Sam sequence of genes. One copy is paternal and the other is maternal; they come from two parents
What is Oogenesis?
The formation of eggs
What is spermatogenesis?
The formation of sperm
What are gametes
Egg and sperm
What are diploid and haploid cells?
Cells of the body are diploid cells: they have two sets of each chromosome (2n) Egg & Sperm have a haploid nucleus: one set of each chromosome (1n)
Function of Meiosis
Meiosis reduces the diploid number of chromosomes (in humans 46) to the haploid number of chromosomes(23)
Why is Meiosis called a reduction division?
Because a diploid parental cell (immature egg or sperm) divided into four haploid cells (2n>>1n)
How does meiosis work?
One replication of chromosomes Two cell divisions(meiosis I and meiosis II)
What are features of meiosis
Crossing over Independent assortment
What happens during interphase?
Interphase: before meiosis begins The chromosomes replicate to produce two copies of each homologous chromosome. The two copies are called sister chromatids, attached to each other by a centromere
What happens during Meiosis I: early Prophase I?
- Chromosomes condense; become visible 2. Nuclear membrane disappears 3. Centrosomes duplicate; move to opposite poles of cell 4. Spindle fibers start to form
What happens during late prophase I?
- Homologous chromosomes pair up together 2. They are in continuous contact in their lengths 3, while they are in contact they exchange segments of chromosomes (Crossing over)
Describe the process of crossing over
DNA on non-sister chromatids break at corresponding points among their length; DNA on on sister chromatids reattaches; As a result, genes are swapped between non-sister chromatids. (During crossing over, broken ends rejoins at corresponding sites)
What is Chiasma
The point where two different chromatids on homologous chromosomes break and exchange a piece is shaped like a cross
Synonym of crossing over
Recombination: the re-assortment of genes into new combinations
What does crossing over do
It increases the genetic variation by the recombination of genes; It can occur in random places along the chromosome at several different positions, thus it can produce an infinite amount of combinations of genes
What happens during Metaphase I
The homologous pairs line, side by side, at the equator; Spindle fibers attach to centromeres
What is independent assortment
Random orientation of the homologous pairs can occur; each pair may orient with its maternal or paternal homolog closer to a given pole; Thus, which chromosome goes to which pole is completely random
What does independent assortment lead to
It leads to genetic variation, as genes on different chromosomes are re-assorted
Why are cells generated by meiosis genetically different from each other
Crossing over and independent assortment
What happens during Anaphase I
Spindle fibers shorten, Homologous pairs are separated and pulled to opposite poles; Sister chromatids remain attached to the centromeres
What type of a division is Anaphase I
A reduction division
What happens during Telophase I?
Nuclear membrane form and cytoplasm begins cytokinesis Spindle fibers break Nucleus is no longer diploid; each nucleus contains one pair of sister geomatics for each chromosome (haploid set) If crossing over has occurred, sister chromatids are no longer identical
What happens during Prophase II(meiosis II)?
The sister chromatids are attached by their centromeres; The nuclear membrane breaks down again, chromosomes shorten and condense;’centrioles move to the opposite pile of the cell; Spindle fibers start forming
What happens during Metaphase II?
Each chromosome, consisting of one pair of sister chromatids, line up at the equator attached by their centromere;each chromosome is attached to now the fully formed spindle
What happens during Anaphase I?
The centromeres separate, and the two chromatids move to opposite poles on the spindle The separated chromatids are now called chromosomes
What happens during Telophase II& cytokinesis
A nuclear envelope forms around each set of chromosomes: 4 nuclei are formed; chromosomes uncoil Cytokinins takes place, producing four daughter cells each with a haploid set of chromosomes Because of crossing over, some chromosomes have recombined segments of the original parental chromosomes
Properties of spermatogenesis
-begins at puberty -occurs in the testes The products of spermatogenesis are four haploid cells (Sperm); cytoplasm is equally divide amount four sperm. Each Sperm is 1/4 of the original diploid cell
Properties of Oogenesis
Occurs in ovary In the human female, the first part of meiosis begins in the embryo. Until puberty, oocytes are in prophase I; After that groups of oocytes periodically resume meiosis
Process of Oogenesis
The products of Oogenesis are one haploid egg And three polar bodies Egg is the same size of original diploid cell Polar bodies are not used and degenerate
Why is the shape of the chromosome characteristic?
Size of the arms depends on the position of the centromere
What is karyotyping?
the examination of the chromosomes number structure
How do you obtain a karyotype?
Chromosomes are isolated, stained, and examined under the microscope. A picture of the chromosomes is taken through the microscope. The picture of the chromosomes is cut up and chromosomes are arranged as homologous pairs in descending order of size.
How are chromosomes organized?
Based on their size in pairs of 23
On what tissues can karyotyping be performed?
Amniotic fluid, Placenta, Blood and Bone Marrow
What does amniocentesis allow
prenatal diagnosis of chromosome abnormalities gender determination
What is non-disjunction
If meiosis does not occur properly in one of the parents, the resulting gamete has an abnormal number of chromosomes; this error in meiosis is called nondisjunction
What is the result of non-disjunction
Formation of gametes with too many or too little chromosomes, baby is born with genetic diesease
What are defective gametes
aneuploid
What types of chromosome abnormalities does nondisjunction lead to
Trisomy and Monosomy
What is trisomy
a third chromosome of one type is present
What is monosomy
one chromosome is missing.Monosomies are usually lethal during early fetal development. Only known survivably monosomy is Turner Syndrome
What happens when there is a nondisjunction of chromosome 21 during egg formation
Down syndrome (trisomy 21), individual has 3 copies of chromosome 21 (47 chromosomes) Mental retardation and distinct physical traits; Probability increases with age of the mother Under 20 years: risk about 1:1700 20-30 years: risk about 1:1400 30-35 years: risk about 1:750 45 years: risk about 1:16
Example of human abnormalities due to the non-disjunction of sex chromosomes
For example: in the mother X chromosomes do not separate during meiosis II; Resulting eggs will be : XX and O
If the XX gamete combines during non-disjunction with an X gamete…
with an X gamete, the resulting zygote is a female XXX, usually sterile (XXX Syndrome);
If the XX gamete combines during non-disjunction with a Y gamete…
with a Y gamete, the resulting zygote is XXY, sterile male with enhanced female characteristics (breast) and diminished mental capacities (Klinefelter syndrome
If a O gamete combines (X gamete)…
with an X gamete, the resulting zygote is XO, sterile female, does not mature sexually (Turner syndrome)
If a O gamete combines (y gamete)..
with a Y gamete, the resulting zygote is nonviable;
Cause of human abnormalities
alterations in sex chromosome number
What happens when an XY chromosome joins with an Y gamete
the resulting zygote is XYY, fertile male, with learning difficulties and violent tendencies
