sexual reproduction and meiosis Flashcards
spermatogenetis
SPERMATOGENESIS
1. Diploid primordial germ cell divide mitotically to produce diploid cells called SPERMATOGONIA (each one can go to mitosis to create a bunch) 2. Each spermatogonium can start meiosis I and do prophase I. Result= primary spermatocyte (2n) ** it is still diploid because the homologous chromosome have not separed yet (happen at the anaphase I) 3. They complete meiosis I, so result= 2 haploids SECONDARY SPERMATOCYTES 4. Meiosis II, result: 4 haploids spermatids * * then they will mature and develop into sperm
SO each primary spermatocytre produces a total of 4 haploid spermatids (compare to one for oogenesis)
oogenesis
OOGENESIS
1. Oogonium (2n) enter prophase I, result= primary oocyte (2n)
2. Complete meiosis I
** it differs from spermatogenesis because after the meiosis I, the cytokenesis is unequal: most of the cytoplasm is allocated to one of the 2 haploids called the SECONDARY OOCYTE. The smaller cell is called the FIRST BODY POLAR (may or may not divide further)
MEIOSIS II: secondary ovocyte do it but again, cytokenesis is unequal: create a bigger one called OVUM and a smaller one called SECOND POLAR BODY
** onlt the ovum can be fertilized and the second polar body is generally disintegrated, so oogenesis produce one single mature gamete from each oocyte
meiosis in plants
- Through meiosis, the diploid (2n) SPOROPHYTE produces haploid (1n) SPORES which become gametophytes
- Through mitosis, the gametophytes produce haploid gametes
- The gametes fuse during fertilization to form a diploid zygote
- so its a life cycle that includes 2 distinct structures/generations:
- A multicellular diploid SPOROPHYTE and a multicellular haploid GAMETPHYTE
steps of meiosis I and II
- Through meiosis, the diploid (2n) SPOROPHYTE produces haploid (1n) SPORES which become gametophytes
- Through mitosis, the gametophytes produce haploid gametes
- The gametes fuse during fertilization to form a diploid zygote
- so its a life cycle that includes 2 distinct structures/generations:
- A multicellular diploid SPOROPHYTE and a multicellular haploid GAMETPHYTE
crossing over
- One chromosome possesses the A and B alleles and the homologous chromosome possesses the a and b alleles
- DNA replication in the S phase produces identical sister chromatids
- During crossing over in prophase I, segments of nonsister chromatids are exchanged
** After the cross-over took place, the chromatid sisters are no longer identical:
One chromatid has A and B, the other one (who underwent crossing over) has a and B
So the four chromatids are: AB, aB, Ab, ab so they all carry an unique combination!!
After meiosis I and II, each of the resulting cells carries a unique combination of alleles
2 processes that contribute to genetic variation
- crossing over (happen on the same chromosome)
2. random separation of homologous chromosomes during the anaphase I (happen on different chromosomes)
random distribution of homologous chromosomes
The process that helps to the variation is the random distribution of chromosome in anaphase I after their random aligment in metaphase I
- In anaphase I, the chromosomes of each homologous pair separate * * how each pair of homologs aligns and separates is random and independent of how other pairs align and separate
The number of possible combinations is 2^n, where n equals the number of homologous pairs. As the number of chromosome pairs increase, the number of combinations because very large
Ex: in that case, n=3 because there are 3 homologous pairs. So, 2^3=8 8 different gametes at the end
similarities and differences in mitosis and meiosis
SIMILARITIES:
1. The chromosomes condense and become visible 2. Both include the movement of chromosomes toward the spindles poles 3. Both are accompanied by cell division
DIFFERENCES:
1. Mitosis: results in a single cell division and usually produces 2 daughter cells
Meiosis: comprises 2 cells division and produces 4 cells
** in diploid cells, homologous chromosomes are present before meiosis and mitosis, but the pairing of homologs take place only in meiosis ????
2. Meiosis: chromosome number is reduced by half (because of the separation of the homologous pairs of chromosomes in anaphase I) Mitosis: no chromosome reduction takes place 3. Meiosis: characterized by 2 processes that produce genetic variation: crossing (prophase I) and random distribution of maternal and paternal chromosome (in anaphase I). Mitosis: nothing 4. Differ in the behaviour of chromosome in methapahse and anaphase: Meiosis: in metaphase I, homologous pairs of chromosomes line up on the metaphase plate. In anahase I, paired chromosomes (each possessing 2 chromatids attached at the centromere) separate and migrate to the opposite poles Mitosis: individual chromosomes line up in metaphase. In anaphase, sister chromatids separate and each chromosome that moves toward the pole is unreplicated
3 reasons why Gregor mendel was successful
- His choice of experimental subject, the pea plant Pisum Sativum offers clear advantages:
- It is easy to cultivate and he had greenhouse at his disposal
- Peas grow rapidly, completing an entire generation in a single growing season
- Peas produce many offspring - their seeds- which allow him to detect meaningful mathematical ratios in the traits he observed in the progeny
- The numerous varieties of peas at his disposition were crucial because these varieties differed in various traits and were genetically pure. So he was able to begin with plants of variable, known genetic makeup
- His success may e because of the 7 characteristics that he chose to study. He avoided characteristics that display a range of variation; instead he focused his attention on those that exist in 2 easily differentiated forms (ex: white vs gray, round vs wrinkles)
- He uses mathematics so he formulated hypothesis based on observations, kept ratios, details…
7 characteristics pea plant of mendel
- Seed (endosperm) color: yellow or green
- Seed shape (round or wrinkled)
- Seed coat color: gray or white
- Pod color: yellor or green
- Pod shape: inflated or constricted
- Flower position (axial= along stem, or terminal= at tip of stem)
- Stem lenght= short or tall
definition gene, allele, locus, genotype, homozygote, heterozygote, phenotype, characteristics
GENE: a genetic factor (region of DNA) that helps determine a characteristic
ALLELE: one of two or more alternate forms of a gene
LOCUSE: specific place on a chromosome occupied by an allele
GENOTYPE: set of alleles possessed by an individual organism
HETEROZYGOTE: an individual organism possessing 2 different alleles at a locus
HOMOZYGOTE: an individual organism possessing 2 of the same alleles at a locus
PHENOTYPE OR TRAIT: the appearance or manifestation of a character
CHARACTER OR CHARACTERISTICS: an attribute or feature
genotype vs phenotype
** PHENOTYPE: can be:
1. Physical
2. Psychological
3. Biochemical
4. Behavioral
Ex: body weight of 50kg, having anemia..
GENOTYPE VS PHENOTYPE:
A given phenotype arises from a genotype that develops within a particular environment. The genotype determines the potential for development: it sets certain limits. How the phenotype develops within those limits is determined by the effects of other genes and of ENVIRONMENTAL FACTORS.
Ex: for pea plant, the phenotype is largely determined by the genotype
Ex: for tree, the phenotype is very influence by the environment (tree will depend on the nutrients, water available)
4 conclusions of Mendel for monohybrid cross (experience with seeds)
- One character is encoded by 2 genetic factos
- 2 genetic factors (alleles) separate when gametes are formed
- The concept of dominant and recessive traits
2 alleles separate with equal probability into gametes
steps of the experience of mendel (monohybrid crosses)
- To cross different varieties of peas, Mendel removed the anthers from flowers to prevent self-fertilization
- And dusted the sigma with pollen from a different plant
- The pollen fertilized ova, which develop into seeds
- The seeds grew into plants
- Mendel crossed 2 homozygous varieties of peas (rounded and wrinkled) = P GENERATION
- All the F1 seeds were round. Mendel allows plants grown from these seeds to self-fertilize= F1 GENERATION
- 3/4 of F2 seeds were round and 1/4 were wrinkled, a 3:1 ratio
segregation (mendel’s first law) + independent assortment (mendel second law)
Each individual diploid organism possesses 2 alleles for any particular characteristic. These 2 alleles segregate when gametes are formed, and 1 allele goes into each gamete
- Each individual organism possesses 2 alleles encoding a trait. BEFORE MEIOSIS
- Alleles separate when gametes are formed (ANAPHASE I)
- Alleles separate in equal proportions (ANAPHASE 1)
Independent:
when 2 different alleles are present in a genotype, only the trait encoded by one of them (the domainant allele) is observed in the phenotype
Alleles at different loci separate independently (ANAPHASE I)
