Lecture 6: Cytogenetics 2 Meiosis in animals, plants and Nurospora Flashcards
Mitosis and Meiosis - Background: How do Chromosomes Pair, Morphological, Mitosis and how do they separate/move?
- Chromosomes occur in pairs,
- one each from maternal and paternal sources - Chromosomes have a morphological
the individuality that is retained through repeated cell divisions - During mitosis chromosomes split longitudinally
- with each half migrating to opposite poles and into respective nuclei of the two daughter cells arising from the division of a pre-existing cell - This guarantees that each daughter cell received the full complement of chromosomes of maternal and paternal origin – mitosis!
Meiosis - Sexual Reproduction: Diploid and homologous
- A DIPLOID organism has 2 sets of Chromosomes organised as HOMOLOGOUS pairs.
- These two versions of a gene encode a trait such as Hair colour.
eg. Allele A and Allele a.
HOMOLOGOUS:
(of chromosomes) pairing at meiosis and having the same structural features and pattern of genes.
DIPLOID:(of a cell or nucleus) containing two complete sets of chromosomes, one from each parent.
The process of meiosis - names very simplified
MEIOSIS I : REDUCTION DIVISION
2n
MEIOSIS II: EQUATIONAL DIVISION
n, n
What happens during Meiosis i:
During Meiosis I, the process of reduction division takes place,
**reducing the chromosome number from diploid to haploid.
- Prophase I:
- Homologous chromosomes pair up and form bivalents or tetrads.
- Crossing over occurs, where segments of genetic material are exchanged between non-sister chromatids.
- Nuclear envelope breaks down, and spindle fibres start to form.
CHROMATIDS (SISTER AND NONSISTER) - Metaphase I:
- Bivalents align along the metaphase plate.
- Spindle fibres from opposite poles attach to each homologous chromosome pair.
HOMOLOGOUS CHROMOSOMES - Anaphase I:
- Homologous chromosomes separate and move toward opposite poles.
- Sister chromatids remain attached at their centromeres.
HOMOLOGOUS CHROMOSOME - Telophase I:
- Chromosomes reach the poles and - decondensed.
- Nuclear envelope may or may not reform.
- Cytokinesis occurs, resulting in the formation of two haploid daughter cells.
2 NEW NUCELI - Interkinesis:
- A brief period of rest between Meiosis I and Meiosis II.
- Chromosomes may be decondensed, and the nuclear envelope may reform.
- No DNA replication occurs during this phase.
PROPHASE OF MEIOSIS I:
- leptotene, zygotene, Pachytene, Diplotene, Diakinesis.
- Leptotene:
Chromosomes condense and become visible as thread-like structures.
Homologous chromosomes begin to pair up. - Zygotene:
Homologous chromosomes continue to pair up and align precisely.
The synaptonemal complex forms between homologous chromosomes, holding them together. - Pachytene:
Homologous chromosomes fully synapse, forming bivalents or tetrads.
Crossing over occurs, where genetic material is exchanged between non-sister chromatids.
Chiasmata (plural of chiasma) are formed at the points of crossover. - Diplotene:
Synaptonemal complex starts to dissolve, except at the chiasmata.
Homologous chromosomes remain attached at the chiasmata.
The chromosomes start to separate slightly. - Diakinesis:
Further condensation of chromosomes occurs.
The nuclear envelope breaks down completely.
Spindle fibres begin to form.
Chromosomes pair - Synaptonemal complex - chiasmata = crossing over
- Chromosomes Pairing:
- Homologous chromosomes come together and align with each other.
- This pairing process is essential for genetic recombination and ensures proper segregation during Meiosis. - Synaptonemal Complex Formation:
- The synaptonemal complex forms between paired homologous chromosomes.
- It is a protein structure that holds the homologous chromosomes tightly together. - Crossing Over:
- During synapsis, genetic material is exchanged between non-sister chromatids of homologous chromosomes.
- This exchange is known as crossing over and leads to the creation of genetic diversity in offspring. - Chiasmata:
- Chiasmata are the sites where crossing over occurs.
- They are visible points of overlap between homologous chromosomes.
- Chiasmata play a role in holding the chromosomes together until separation in Anaphase I.
Explain the stages of gametogenesis: oogenesis: Oogonium Stage: (2n), Primary Oocyte Stage: 2n, Secondary Oocyte Stage: (1n), First Polar Body,Ovum Stage: Second polar body
- Oogonium Stage: (2n)
- Diploid cells called oogonia undergo mitotic divisions.
- Each oogonium contains 46 chromosomes (23 pairs) and 46 chromatids. - Primary Oocyte Stage: 2n
- Oogonia develop into primary oocytes.
- Each primary oocyte contains 46 chromosomes and 92 chromatids.
- They are arrested in prophase I of Meiosis I until puberty. - Secondary Oocyte Stage: (1n)
- During ovulation, a primary oocyte completes Meiosis I.
- This division results in the formation of a secondary oocyte and a polar body.
- The secondary oocyte receives half of the chromosomes (23) and 46 chromatids. - First Polar Body:
- The polar body contains half of the chromosomes (23) and 46 chromatids.
- It may undergo a second division, resulting in two polar bodies. - Ovum Stage: Second polar body
- If fertilization occurs, the secondary oocyte progresses to the ovum stage.
- The ovum contains 23 chromosomes and 23 chromatids.
- It is the mature, haploid female gamete ready for fertilization.
- In some animals, meiosis 2 takes place after sperm has penetrated the secondary oocyte.
A sperm and ovum fuse at fertilisation to produce a DIPLOID ZYGOTE.
Explain the stages of gametogenesis: SPERMATOGENESIS: Spermatogonium Stage, Primary Spermatocyte Stage: (2n), Secondary Spermatocyte Stage: (1n), Spermatids Stage: (1n),Spermatozoa (Sperm) Stage:
- Spermatogonium Stage:
- Diploid cells called spermatogonia undergo mitotic divisions in the testes.
- Each spermatogonium contains 46 chromosomes (23 pairs) and 46 chromatids. - Primary Spermatocyte Stage: (2n)
- Spermatogonia develop into primary spermatocytes.
- Each primary spermatocyte contains 46 chromosomes and 92 chromatids.
- They are arrested in prophase I of Meiosis I. - Secondary Spermatocyte Stage: (1n)
- During Meiosis I, primary spermatocytes divide into secondary spermatocytes.
- Each secondary spermatocyte contains 23 chromosomes and 46 chromatids. - Spermatids Stage: (1n)
- Secondary spermatocytes further divide through Meiosis II, forming spermatids.
- Each spermatid contains 23 chromosomes and 23 chromatids. - Spermatozoa (Sperm) Stage:
- Spermatids undergo a process called spermiogenesis to differentiate into spermatozoa. –>
MEIOSIS 2
- Spermatozoa are mature sperm cells with 23 chromosomes and 23 chromatids.
- They have a distinct head (containing genetic material), midpiece (with mitochondria for energy production), and tail (for motility).
A sperm and ovum fuse at fertilisation to produce a DIPLOID ZYGOTE.
Explain the formation of gametes in animals (2n to n),
individual, meiois, gametes, fertilisation, zygote
- Individual Stage:
- Animals start as diploid individuals, meaning they have two sets of chromosomes (2n).
- Each cell in the body carries a complete set of chromosomes. - Meiosis Stage:
- Meiosis is a specialized cell division process that occurs in the reproductive organs (gonads) to produce gametes.
- It involves two rounds of division: Meiosis I and Meiosis II. - Gametes Stage:
- Meiosis I: During this phase, diploid cells called germ cells (spermatocytes in males, oocytes in females) undergo division.
- Each germ cell divides into two haploid cells called secondary spermatocytes or secondary oocytes.
- Each secondary spermatocyte or secondary oocyte contains half the number of chromosomes (n) but still has replicated chromatids. - Meiosis II:
- Meiosis II follows Meiosis I and is similar to mitosis.
- The two secondary spermatocytes or secondary oocytes divide further.
- In males, each secondary spermatocyte divides into two spermatids, resulting in four haploid spermatids.
- In females, only one secondary oocyte continues to Meiosis II and divides into one mature egg (ovum) and a second polar body. - Fertilization:
- Fertilization is the process of union between a sperm cell and an egg cell (ovum).
- It occurs when a sperm penetrates the egg, combining its genetic material.
- Fertilization restores the diploid number of chromosomes (2n) in the resulting zygote. - Zygote:
- The zygote is the first cell of the new individual formed by the fusion of sperm and egg.
- It is diploid (2n) since it contains a complete set of chromosomes from both parents.
- The zygote will undergo cell division and develop into an organism.
Explain the formation of gametes in plants (2n to n)
individual (sporophyte), meiosis, spores, mitosis, individual (gametophytes), gametes, fertilization, zygote, mitosis,
- Individual Stage (Sporophyte):
- Plants start as diploid individuals called sporophytes, which possess two sets of chromosomes (2n).
- The sporophyte is the dominant phase of the plant’s life cycle. - Meiosis Stage:
- Meiosis is a specialized cell division process that occurs in the sporophyte’s reproductive organs (sporangia or cones).
- During meiosis, diploid cells called sporocytes undergo division to produce haploid cells. - Spore Formation:
- Meiosis I: Each sporocyte divides, resulting in four haploid cells called spores.
- Each spore contains half the number of chromosomes (n) as the parent sporocyte. - Mitosis (Gametophytes):
- Each spore undergoes mitotic divisions to develop into a gametophyte.
- Gametophytes are the haploid stage of the plant’s life cycle.
- In flowering plants, the male gametophyte is the pollen grain, and the female gametophyte is the embryo sac. - Gametes Formation:
- Male Gametophyte: The pollen grain produces sperm cells through mitotic divisions.
- Female Gametophyte: The embryo sac develops and contains one or more egg cells. - Fertilization:
- Fertilization occurs when a pollen grain (containing sperm) reaches the female reproductive structure (pistil).
- The sperm cell fertilizes the egg cell, combining their genetic material. - Zygote Formation:
- Fertilization results in the formation of a diploid zygote (2n).
- The zygote develops into a new sporophyte individual. - Mitosis (Sporophyte):
- The zygote undergoes mitotic divisions, growing into a mature sporophyte individual.
- The cycle then repeats, continuing the alternation between sporophyte and gametophyte generations.
explain plant meiosis both sperm and egg.
Sure! Here’s a revised version of the information in easy-to-learn flashcards:
- Microsporocyte Meiosis:
- Diploid microsporocytes in the stamen undergo meiosis.
- Meiosis produces four haploid microspores. - Pollen Grain Formation:
- Each microspore undergoes mitosis to form a pollen grain with two haploid nuclei. - Pollen Tube Growth:
- The tube nucleus within the pollen grain directs the growth of a pollen tube. - Generative Nucleus Division:
- The generative nucleus divides mitotically to produce two sperm cells within the pollen tube. - Megasporocyte Meiosis:
- In the ovary, the diploid megasporocyte undergoes meiosis.
- Meiosis produces four haploid megaspores, but only one survives. - Megaspore Mitosis:
- The surviving megaspore undergoes three rounds of mitosis. - Formation of Eight Nuclei:
- Mitosis results in the production of eight haploid nuclei. - Cytoplasm Division:
- The cytoplasm divides, forming separate cells. - Egg Formation:
- One of the cells becomes an egg. - Polar Nuclei:
- Two of the nuclei become polar nuclei. - Partitioning of Nuclei:
- The other nuclei are partitioned into separate cells. - Double Fertilization:
- Double fertilization occurs when two sperm cells from a pollen grain enter the embryo sac. - Fertilization of Egg:
- One sperm cell fertilizes the egg, forming a diploid zygote. - Formation of Endosperm:
- The other sperm cell fuses with the binucleate cell, resulting in the formation of a triploid endosperm.
Explain Neurospora - a model in genetics (7)
- Red bread mold/mould
- Haploid and diploid life cycle
- Linear arrangement of meiotic product
- Meiosis plus spore mitosis
- Model to look at the cross over
- Model for independent assortment
- Seven chromosomes fully sequenced
Neurospora in detail:
Neurospora crassa, commonly known as bread mold, is a filamentous fungus that has served as a model organism in genetics and molecular biology research for many decades. Here’s an overview of Neurospora and its significance as a genetic model:
- General Characteristics:
- Neurospora crassa is a type of ascomycete fungus that belongs to the group of filamentous fungi.
- It is commonly found on decaying plant materials, especially bread, hence its nickname “bread mold.”
- Neurospora has a relatively simple life cycle and can be easily cultured in the laboratory. - Easy Cultivation:
- One of the reasons Neurospora became a popular genetic model is its fast growth and ease of cultivation.
- It can be grown on simple media containing agar, sugar, and essential nutrients, making it accessible for research purposes. - Haploid Genome:
- Neurospora has a haploid genome, meaning it carries only one copy of each chromosome in its cells.
- This characteristic simplifies the study of genetic inheritance and the identification of mutations. - Mutant Isolation:
- Neurospora displays a wide range of observable and heritable traits, making it suitable for the isolation and study of mutants.
- Researchers have identified numerous mutants affecting traits such as growth rate, pigmentation, nutritional requirements, and enzyme production. - Sexual and Asexual Reproduction:
- Neurospora can reproduce both sexually and asexually, offering researchers different avenues for genetic studies.
- Sexual reproduction involves the fusion of two compatible haploid strains, leading to the formation of a diploid zygote.
- Asexual reproduction occurs through the production of asexual spores called conidia. - Genetic Mapping:
- Neurospora was one of the first organisms in which genetic mapping techniques were developed.
- Researchers have created detailed genetic maps, linking specific traits or mutations to specific regions of the genome. - Biochemical and Genetic Studies:
- Neurospora has been instrumental in elucidating various biochemical and genetic processes.
- Studies on Neurospora have contributed to our understanding of gene regulation, enzymatic pathways, circadian rhythms, and the control of cell division. - The “One Gene-One Enzyme” Hypothesis:
- In the 1940s, Neurospora played a crucial role in the formulation of the “one gene-one enzyme” hypothesis by George Beadle and Edward Tatum.
- Their experiments with Neurospora demonstrated that genes control the production of specific enzymes, laying the foundation for modern molecular genetics.
Neurospora crassa has provided valuable insights into the fundamental principles of genetics and molecular biology. Its ease of cultivation, haploid genome, genetic mappability, and versatility for both sexual and asexual reproduction have made it a powerful model organism for scientific research.
Meiosis and Mitosis in Neurospora: 3.
- Meiosis - four linear tetrads
- Mitosis makes 8 spores
- Markers enable us to see:
- the cross over
- the recombination
Meiosis and Mitosis in Neurospora: in detail
Meiosis in Neurospora crassa:
- Neurospora crassa undergoes meiosis as part of its sexual reproduction process.
- Meiosis occurs when two compatible haploid strains of Neurospora come together to form a diploid zygote.
- The diploid zygote then undergoes meiosis to produce haploid spores.
- Meiosis involves two rounds of cell division, resulting in the reduction of the chromosome number from diploid (2n) to haploid (n).
- This reduction in chromosome number allows for genetic diversity and the exchange of genetic material.
Mitosis in Neurospora crassa:
- Neurospora crassa also undergoes mitosis as part of its asexual reproduction and growth.
- Mitosis is the process by which a single diploid (2n) cell divides to produce two genetically identical diploid daughter cells.
- During mitosis, the replicated chromosomes line up in the center of the cell and are then separated into two daughter nuclei.
- The cytoplasm then divides, and two distinct cells are formed, each containing the same genetic material as the parent cell.
- Mitosis allows Neurospora to grow and multiply through asexual spore production.
Both meiosis and mitosis play important roles in the life cycle of Neurospora crassa. Meiosis is involved in sexual reproduction, ensuring genetic diversity, while mitosis is responsible for asexual reproduction and growth, generating genetically identical offspring.
