Meiosis Overview

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Questions and Answers

What does the term cumulus oophorus refer to in the context of ovarian follicles?

A fluid-filled space within the follicle

The primary oocyte before it enlarges

Inner follicular cells surrounding the oocyte (correct)

The outer layer of the ovary

Which of the following occurs in the Graafian follicle before ovulation?

Formation of multiple antrums within the follicle

Processes of granulosa cells retract from the oocyte surface (correct)

The theca externa becomes a vascular layer

The cumulus oophorus each divides to form polar bodies

What is the role of the zona pellucida in the ovarian follicle?

It supports the development of the cumulus oophorus

It serves as the site for follicular fluid storage

It separates the oocyte from the surrounding granulosa cells (correct)

It assists in the vascularization of the follicle

Which layer of the theca becomes vascular and glandular in nature?

Theca interna Signup and view all the answers

What happens to non-selected tertiary follicles during the maturation process?

They undergo atresia and degenerate Signup and view all the answers

What occurs during metaphase of mitosis?

Chromosomes align at the equatorial plane. Signup and view all the answers

Which stage follows telophase in mitosis?

Cytokinesis Signup and view all the answers

What is the genetic outcome of meiosis compared to mitosis?

Meiosis results in genetically diverse gametes. Signup and view all the answers

During which phase does crossing over occur in meiosis?

Prophase I Signup and view all the answers

What is the chromosome configuration of a cell entering meiosis II?

Haploid with unreplicated chromosomes Signup and view all the answers

What happens during anaphase of mitosis?

Sister chromatids are pulled towards opposite poles. Signup and view all the answers

Which of these statements about meiosis is accurate?

Two successive cell divisions are involved. Signup and view all the answers

What is true about the chromosomes during telophase of meiosis?

They cluster at the poles of the cell. Signup and view all the answers

What is the function of the acrosome in mature sperm?

To penetrate the ova during fertilization Signup and view all the answers

Which of the following statements about the mid piece of sperm is true?

It provides energy for flagellar movement Signup and view all the answers

What is the primary role of the tail in mature sperm?

To provide motility Signup and view all the answers

During which stage of oogenesis does the primary oocyte become surrounded by follicular cells?

Before birth Signup and view all the answers

What characterizes primary oocytes in females after birth?

No new primary oocytes form Signup and view all the answers

Which structure contains the axoneme in the tail of the sperm?

Principal piece Signup and view all the answers

At what point does oogenesis begin?

Before birth Signup and view all the answers

What happens to primary oocytes after they enter prophase I of meiosis?

They become dormant until ovulation Signup and view all the answers

What is the primary reason women reach menopause between the ages of 45 and 50?

Limited number of primary oocytes present at birth Signup and view all the answers

What occurs to primary oocytes during the prenatal period until puberty?

They remain dormant in primordial follicles Signup and view all the answers

What factor inhibits the completion of meiosis I in primary oocytes during the early stages of development?

Oocyte maturation inhibitor factor Signup and view all the answers

What is the role of granulosa cells in the ovarian follicle?

Nourish the oocyte through yolk supply Signup and view all the answers

At what stage is the secondary oocyte arrested before fertilization?

Metaphase II Signup and view all the answers

What characterizes the transition from primary to secondary follicle?

Development of multiple layers of follicular cells Signup and view all the answers

What happens to the first polar body after meiotic division of the primary oocyte?

It undergoes apoptosis Signup and view all the answers

What defines the primordial follicle in the ovary?

A single layer of flat follicular cells Signup and view all the answers

What is the primary characteristic of the animal pole in an ovum?

Characterized by active, rapid growth Signup and view all the answers

What defines a polylecithal ovum?

Multiple yolk bodies distributed throughout Signup and view all the answers

What occurs during spermatogenesis after birth?

Formation of spermatogonia types A and B Signup and view all the answers

During oogenesis, when is the second meiotic division completed?

At fertilization Signup and view all the answers

What distinguishes the developmental pathway of sperm from that of an ovum?

Spermatogenesis is continuous while oogenesis is limited Signup and view all the answers

Which of the following is true regarding primary oocytes?

All primary oocytes are formed before birth Signup and view all the answers

What is a characteristic phenomenon of spermiogenesis?

Development of four identical sperm from one spermatogonium Signup and view all the answers

How does cell movement differ between spermatogenesis and oogenesis?

Spermatogenesis involves centripetal, oogenesis involves centrifugal movement Signup and view all the answers

In terms of quantity, how do sperm and ovum production compare?

Sperm production is in millions while ovum is typically one Signup and view all the answers

What type of egg is characterized by a moderate amount of yolk and slower division rates?

Mesolecithal Signup and view all the answers

Study Notes

Meiosis

Occurs in germ cells (sex cells)
Creates haploid gametes (1N) like sperms and oocytes
Daughter cells are not genetically identical due to the exchange of genetic material between non-sister chromatids (crossover)
Germ cell enters meiosis with diploid chromosomes (2N)
Two successive meiotic divisions (meiosis I and meiosis II)

Meiosis I

Homologous chromosomes pair
Each chromosome has two chromatids
Crossover can occur, exchanging chromatid segments
Chromosomes pair at the meiotic spindle
Chromatids migrate to opposite poles
Parental chromosome pairs are distributed at the end of the first meiotic division

Meiosis II

Similar to mitosis but cells are haploid
Follows the first division without a normal interphase
Chromosome number entering the second meiotic division is haploid

Chromosome Numbers

Chicken, dog: 39 chromosomes in pairs
Horse: 32 chromosomes in pairs
Cattle, goat: 30 chromosomes in pairs
Sheep: 27 chromosomes in pairs
Human: 23 chromosomes in pairs
Pig, cat: 19 chromosomes in pairs

Sperm Structure

Mature sperms are free-swimming, motile cells with a head, neck, mid-piece, and tail
Head: Contains the haploid nucleus, covered by the acrosome in the anterior two-thirds
Acrosome: Cap-like saccular organelle containing enzymes for fertilization (hyaluronidase)
Neck: Contains two centrioles (proximal and distal), with the distal forming the flagellum
Mid-piece: Contains mitochondria forming a spiral filament around the sheath of the outer fibers and the axoneme, providing energy for flagellar movement
Tail: Provides motility, composed of two pieces:
- Principal piece: Sheath of ring fibers around the axoneme
- End piece: Consists of 9+2 structure of the axoneme

Oogenesis

The process of transforming oogonia into mature oocytes
Begins before birth, completes after puberty, and continues to menopause
Divided into two periods: Prenatal maturation and Postnatal maturation of oocytes

Prenatal Maturation

Occurs before birth
Starts with primordial germ cells (2N) which divide mitotically to give rise to oogonia (2N)
Oogonia proliferate by mitosis and become primary oocytes (2N)
Primary oocytes begin the first meiotic division before birth but are arrested in prophase I
The primary oocyte is surrounded by flat follicular cells, forming the primordial follicle
Primary oocyte formation stops at birth for females, explaining menopause
Each female is born with a limited number of primary oocytes

Postnatal Maturation of Oocytes

After birth: Primary oocytes (arrested in prophase I) remain dormant within primordial follicles
At puberty: Primary oocyte enlarges and the primordial follicle becomes a primary follicle
Hours before ovulation: The primary oocyte completes the first meiotic division to produce a secondary oocyte (1N, large) and the first polar body (1N, small, degenerates)
Before ovulation: Oocyte maturation inhibitor factor arrests meiosis I in the dictyotene stage of prophase I, until puberty when hormones unblock it
At ovulation: The secondary oocyte begins meiosis II, but is arrested in metaphase II
After fertilization: The secondary oocyte completes meiosis II, forming the fertilized oocyte and the second polar body

Folliculogenesis

The growth and development of ovarian follicles from primordial to ovulatory stages
Females are born with a fixed number of follicles
Primordial follicle: Consists of one primary oocyte surrounded by a single layer of squamous follicular cells
Primary follicle: Follicular cells around the primary oocyte become cuboidal
Secondary follicle:
- Granulosa cells: Follicular cells proliferate into stratified granulosum
- Zona pellucida: Translucent glycoprotein layer between the oocyte and follicular epithelium
  - Provides communication and nourishment to the primary oocyte
- Theca follicular cells: Formed by the ovarian stroma surrounding the basal lamina of granulosa cells
Tertiary (antral) follicle:
- Fluid-filled spaces fuse to form the antrum
- The primary oocyte is pushed to one side of the follicle
- Cumulus oophorus: Inner follicular cells surrounding the oocyte
- Corona radiata: Radially arranged cells on the oocyte cell membrane
- Theca interna: Vascular and glandular layer
- Theca externa: Fibrous layer
Graafian (mature) follicle:
- Selected tertiary follicle that continues to enlarge until maturity
- Atresia: Non-selected tertiary follicles degenerate
- Changes before ovulation:
  - Spindle forms and orients radially
  - Granulosa cells retract from the oocyte surface, forming the perivitelline space
  - First polar body is ejected, indicating the end of meiosis I and formation of the secondary oocyte

Types of Ova

Amount of yolk:
- Oligolecithal: No demarcation line between yolk and cytoplasm (lower fishes and frogs)
- Mesolecithal: Isolation of yolk (demarcation line) (frogs)
- Polylecithal: Yolk uniformly distributed with cytoplasm, but there is a demarcation line (birds and reptiles)
Distribution of yolk:
- Telolecithal: Yolk concentrated at one pole (birds, reptiles, frogs)
- Centrolecithal: Yolk in the center (insects)

Egg (Ovum) Coverings

Animal pole: Contains cytoplasm and the nucleus, and is characterized by rapid growth
Vegetable pole: Contains yolk and is characterized by slow growth
Cleavage divisions: Occur rapidly at the animal pole, forming the embryo
Yolk sac, placenta, and allantois: Form from the vegetable pole

Spermatogenesis vs. Oogenesis

Occurrence: Spermatogenesis in seminiferous tubules of the testes, oogenesis in the ovary cortex
Start time: Both start before birth
Direction of cell movement: Spermatogenesis is centripetal, oogenesis is centrifugal
Proliferation (mitosis): Spermatogenesis starts late but continues until death, oogenesis ends before birth
- Spermatogonia can give rise to primary spermatocytes after birth
- All primary oocytes are formed before birth
- Spermatogonia can divide homonymously and heteronymously
- Oogonia only divide homonymously
Meiosis I: Spermatogenesis occurs regularly, oogenesis starts before birth and completes hours before ovulation
- Spermatogenesis results in two equal size secondary spermatocytes
- Oogenesis results in one large secondary oocyte and one small polar body
Meiosis II: Spermatogenesis occurs regularly, oogenesis starts at ovulation and is completed in the uterine tube after fertilization
- Spermatogenesis results in four equal size sperms with either X or Y chromosomes
- Oogenesis results in a fertilized oocyte and a second polar body
Transformation: Spermiogenesis (transformation of spermatids into sperm) occurs, not in oogenesis
End result: One spermatogonium produces four sperms, one oogonium produces one ova and three smaller polar bodies

Comparison of Sperm and Ovum

Size: Sperm is small, ovum is large
Shape: Sperm is elongated, ovum is spherical
Quantity: Millions of sperms, 1-25 ova

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Description

Explore the fascinating process of meiosis, which occurs in germ cells to produce haploid gametes. Learn about the two successive divisions, meiosis I and meiosis II, and how genetic variation is achieved through crossover. This quiz covers key concepts, including chromosome numbers in various species.