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1. Introduction to Biology2h 42m
2. Chemistry3h 37m
3. Water1h 26m
4. Biomolecules2h 23m
5. Cell Components2h 26m
6. The Membrane2h 31m
7. Energy and Metabolism2h 0m
8. Respiration2h 40m
9. Photosynthesis2h 49m
10. Cell Signaling59m
11. Cell Division2h 47m
12. Meiosis2h 0m
13. Mendelian Genetics4h 44m
14. DNA Synthesis2h 27m
15. Gene Expression3h 6m
16. Regulation of Expression3h 31m
17. Viruses37m
- Viruses
  37m
18. Biotechnology2h 58m
19. Genomics17m
- Genomes
  17m
20. Development1h 5m
21. Evolution3h 1m
22. Evolution of Populations3h 53m
23. Speciation1h 37m
24. History of Life on Earth2h 6m
25. Phylogeny2h 31m
26. Prokaryotes4h 59m
27. Protists1h 12m
28. Plants1h 22m
29. Fungi36m
- Fungi
  19m
- Fungi Reproduction
  17m
30. Overview of Animals34m
- Overview of Animals
  34m
31. Invertebrates1h 2m
32. Vertebrates50m
33. Plant Anatomy1h 3m
34. Vascular Plant Transport1h 2m
- Water Potential
  1h 2m
35. Soil37m
- Soil and Nutrients
  20m
- Nitrogen Fixation
  16m
36. Plant Reproduction47m
- Flowers
  33m
- Seeds
  14m
37. Plant Sensation and Response1h 9m
38. Animal Form and Function1h 19m
39. Digestive System1h 10m
- Digestion
  1h 3m
- Blood Sugar Homeostasis
  7m
40. Circulatory System1h 49m
41. Immune System1h 12m
42. Osmoregulation and Excretion50m
- Osmoregulation and Excretion
  50m
43. Endocrine System1h 4m
- Endocrine System
  1h 4m
44. Animal Reproduction1h 2m
- Animal Reproduction
  1h 2m
45. Nervous System1h 55m
- Neurons and Action Potentials
  1h 8m
- Central and Peripheral Nervous System
  46m
46. Sensory Systems46m
- Sensory System
  46m
47. Muscle Systems23m
- Musculoskeletal System
  23m
48. Ecology3h 11m
49. Animal Behavior28m
- Animal Behavior
  28m
50. Population Ecology3h 41m
51. Community Ecology2h 46m
52. Ecosystems2h 36m
53. Conservation Biology24m
- Conservation Biology
  24m

12. Meiosis

Meiosis I

12. Meiosis

Meiosis I: Videos & Practice Problems

Video Lessons Practice Worksheet

Topic summary

Meiosis 1 begins after interphase, which includes G1, S, and G2 phases, where DNA replication occurs. Meiosis 1 consists of prophase 1, metaphase 1, anaphase 1, telophase 1, and cytokinesis, similar to mitosis but with key differences. In metaphase 1, homologous chromosomes align in two rows, while in anaphase 1, homologous chromosomes separate, leaving sister chromatids intact. Cytokinesis results in two haploid daughter cells, each with half the original chromosome number, preparing for meiosis 2.

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Meiosis I

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Meiosis I Video Summary

Meiosis is a crucial process in sexual reproduction, and it begins with meiosis I, which follows the interphase stage. Interphase consists of three phases: G1, S, and G2. During the S phase, DNA replication occurs, ensuring that each chromosome is duplicated before meiosis begins. After interphase, the cell enters meiosis I, which includes distinct phases: prophase I, metaphase I, anaphase I, telophase I, and cytokinesis.

Meiosis I shares similarities with mitosis, particularly in the structure of its phases. However, it is essential to note that the phases of meiosis I are denoted with a "1" (e.g., prophase I, metaphase I). While prophase I and telophase I exhibit events similar to those in mitosis, the most significant differences arise during metaphase I and anaphase I.

In metaphase I, homologous chromosomes align in the center of the cell in two rows, contrasting with the single-file alignment seen in mitosis. This arrangement allows for the pairing of homologous chromosomes, which is a key feature of meiosis. During anaphase I, the homologous chromosomes separate and move toward opposite poles of the cell, while the sister chromatids remain attached. This is a critical distinction from mitosis, where sister chromatids separate.

Following anaphase I, telophase I occurs, leading to cytokinesis, which divides the cytoplasm and results in two haploid daughter cells. Each of these cells contains half the number of chromosomes compared to the original diploid cell, marking a significant reduction in chromosome number. For example, if the original cell had four chromosomes, each haploid daughter cell will have two chromosomes.

This reduction in chromosome number is vital for maintaining genetic stability across generations. The two haploid cells produced from meiosis I will then proceed to meiosis II, where further division occurs. Understanding the differences between meiosis and mitosis is essential for grasping the complexities of genetic variation and reproduction.

Problem

A daughter cell is created by meiosis I and the first round of cytokinesis. This daughter cell is just beginning meiosis II. Which of the following is an appropriate description of this daughter cell's genetic contents?
a) It has half the amount of DNA as the parent cell.
b) It has half the chromosomes but twice the DNA of the parent cell.
c) It has one-fourth the DNA and one-half the chromosomes as the parent cell.
d) It is genetically identical to the parent cell.

It has half the amount of DNA as the parent cell.

It has half the chromosomes but twice the DNA of the parent cell.

It has one-fourth the DNA and one-half the chromosomes as the parent cell.

It is genetically identical to the parent cell.

Problem

Which diagram represents anaphase I of meiosis?

a) I
b) II
c) III
d) IV
e) V
f) VI

III

Dig Deeper into Meiosis I

Meiosis I is the first division in meiosis where homologous chromosomes separate, reducing the chromosome number by half to produce haploid cells.

Key Terminology

Interphase: The phase before meiosis where the cell grows and DNA is replicated during the S phase.
Prophase 1: The first stage of meiosis I where chromosomes condense and homologous chromosomes pair up.
Metaphase 1: The stage where homologous chromosomes align in two rows at the cell’s equator, unlike mitosis where chromosomes line up in a single row.
Anaphase 1: The phase in meiosis I where homologous chromosomes separate and move to opposite poles, but sister chromatids remain connected.
Telophase 1: The stage where chromosomes arrive at the poles and the cell prepares to divide.
Cytokinesis: The division of the cytoplasm following telophase, resulting in two haploid daughter cells.
Haploid: A cell containing one set of chromosomes, half the diploid number.
Diploid: A cell containing two sets of chromosomes, one from each parent.
Sister chromatids: Two identical copies of a replicated chromosome connected at the centromere.
Homologous chromosomes: Chromosome pairs, one from each parent, that are similar in shape, size, and gene content.
Chromosome: A structure of DNA and protein that carries genetic information.
Replication: The process of copying DNA during the S phase of interphase.
Cell cycle: The series of phases a cell goes through including interphase and mitotic or meiotic phases.

Real-World Applications

Understanding meiosis I is crucial in genetics and reproductive biology, especially in explaining how genetic variation arises through the separation of homologous chromosomes.
Knowledge of meiosis I helps in medical fields such as genetic counseling and prenatal testing, where errors in chromosome separation can lead to conditions like aneuploidy.
Research on meiosis I informs breeding programs in agriculture and animal husbandry by manipulating genetic recombination and inheritance patterns.

Common Misconceptions

Many people think sister chromatids separate during meiosis I, but actually, it’s the homologous chromosomes that separate while sister chromatids stay together until meiosis II.
It’s easy to confuse metaphase 1 of meiosis with metaphase of mitosis; however, in meiosis I, chromosomes line up in two rows (pairs of homologs), not a single row.
Some assume meiosis immediately produces haploid cells, but the haploid state is achieved only after cytokinesis following meiosis I, not before.
Another common mix-up is thinking meiosis I and mitosis are completely different; in fact, prophase 1 and telophase 1 share many similarities with mitotic phases.

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Here’s what students ask on this topic:

What are the main phases of Meiosis I and how do they differ from Mitosis?

Meiosis I consists of prophase I, metaphase I, anaphase I, telophase I, and cytokinesis. While prophase I and telophase I are similar to their mitotic counterparts, metaphase I and anaphase I differ significantly. In metaphase I, homologous chromosomes align in two rows at the cell's equator, unlike the single row in mitosis. During anaphase I, homologous chromosomes separate, but sister chromatids remain connected, whereas in mitosis, sister chromatids separate. These differences are crucial for reducing the chromosome number by half, resulting in haploid cells.

What happens during the S phase of interphase before Meiosis I?

During the S phase of interphase, DNA replication occurs, resulting in the synthesis of identical sister chromatids for each chromosome. This replication is essential for ensuring that each daughter cell receives the correct amount of genetic material. The S phase is followed by the G2 phase, where the cell prepares for meiosis. Only after completing interphase can the cell enter Meiosis I, where the replicated chromosomes will undergo further segregation.

How do homologous chromosomes behave differently in Metaphase I of Meiosis I compared to Mitosis?

In Metaphase I of Meiosis I, homologous chromosomes pair up and align themselves in two rows at the cell's equator. This arrangement contrasts with mitosis, where individual chromosomes align in a single row. The pairing of homologous chromosomes in two rows is crucial for the subsequent separation of homologous chromosomes during Anaphase I, which reduces the chromosome number by half in the resulting daughter cells.

What is the significance of cytokinesis in Meiosis I?

Cytokinesis in Meiosis I is the process that divides the cytoplasm of the parent cell into two haploid daughter cells. This division is significant because it ensures that each daughter cell receives half the original chromosome number, setting the stage for Meiosis II. The resulting haploid cells contain one set of chromosomes, which is essential for maintaining genetic diversity and stability in sexually reproducing organisms.

Why do sister chromatids remain connected during Anaphase I of Meiosis I?

During Anaphase I of Meiosis I, sister chromatids remain connected because it is the homologous chromosomes that are separated and pulled to opposite poles of the cell. This connection ensures that each daughter cell receives one chromosome from each homologous pair, maintaining the correct chromosome number. The separation of sister chromatids occurs later in Meiosis II, ensuring that each gamete receives a single copy of each chromosome.