Meiosis Notes, Lecture notes of Reasoning

Download Meiosis Notes and more Lecture notes Reasoning in PDF only on Docsity!

Meiosis Notes

THINK ABOUT IT

As geneticists in the early 1900s applied Mendel’s laws, they wondered where genes might be located.

They expected genes to be carried on structures inside the cell, but which structures_?_

What cellular processes could account for segregation and independent assortment, as Mendel had described?

What are Chromosomes?

Chromosomes—those strands of DNA

and protein inside the cell nucleus—

are the carriers of genes.

The genes are located in specific positions on chromosomes.

What are Diploid Cells

A body cell in an adult fruit fly has eight chromosomes, as shown in the figure.

Four of the

chromosomes come

from its male parent, and

four come from its

female parent.

These two sets of chromosomes are homologous , meaning that each of the four chromosomes from the male parent has a corresponding chromosome from the female parent.

What are Haploid Cells

Some cells contain only a single set of

chromosomes, and therefore a single set

of genes.

Such cells are haploid , meaning “one

set.”

The gametes of sexually reproducing organisms are haploid.

For fruit fly gametes, the haploid number is 4, which can be written as N = 4.

What is Meiosis?

Meiosis is a process in which the number

of chromosomes per cell is cut in half

through the separation of homologous

chromosomes in a diploid cell.

Meiosis usually involves two distinct divisions, called meiosis I and meiosis II.

By the end of meiosis II, the diploid cell becomes four haploid cells.

Phases of Meiosis

What events occur during each phase of meiosis?

As the cells enter prophase II, their chromosomes—each consisting of two chromatids—become visible.

The final four phases of meiosis II are similar to those in meiosis I. However, the result is four haploid daughter cells.

Phases of Meiosis

What events occur during each phase of meiosis?

During anaphase I, spindle fibers pull each homologous chromosome pair toward opposite ends of the cell.

In telophase I, a nuclear membrane forms around each cluster of chromosomes. Cytokinesis follows telophase I, forming two new cells.

Prophase I

The cells begin to divide, and the chromosomes pair up, forming a structure called a tetrad , which contains four chromatids.

Prophase I

As homologous chromosomes pair up and form tetrads, they undergo a process called crossing-over.

First, the chromatids of the homologous chromosomes cross over one another.

Metaphase I and Anaphase I

As prophase I ends, a spindle forms and attaches to each tetrad.

During metaphase I of meiosis, paired homologous chromosomes line up across the center of the cell.

Metaphase I and Anaphase I

During anaphase I, spindle fibers pull each homologous chromosome pair toward opposite ends of the cell.

When anaphase I is complete, the separated chromosomes cluster at opposite ends of the cell.

Meiosis I

Meiosis I results in two cells, called daughter cells, each of which has four chromatids, as it would after mitosis.

Because each pair of homologous chromosomes was separated, neither daughter cell has the two complete sets of chromosomes that it would have in a diploid cell.

The two cells produced by meiosis I have sets of chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I.

Meiosis II

The two cells produced by meiosis I now enter a second meiotic division.

Unlike the first division, neither cell goes through a round of chromosome replication before entering meiosis II.