Meiosis and Genetic Disorders

Lesson 2.

Learning goals

• Understand the process of Meiosis I and Meiosis II and the ploidy that results

• Understand crossing over and how it relates to increasing genetic diversity

• Understand the errors that can occur during meiosis and identify some disorders using karyotypes

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In Mitosis…

• parent and daughter cells have the same number of chromosomes

• these chromosomes occur in two sets ( , ♀) and the two sets are called ♂ homologus

chromosomes

• the total number of chromosomes is referred to as the diploid number

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MeiosisMeiosis:

• is similar to mitosis in that it is just a single replication of nuclear material

BUT:• this replication is followed by TWO cell

divisions, creating 4 gametes with haploid number of chromosomes

• e.g., each cell only has ONE copy of each chromosome

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MeiosisMeiosis:

• is the division process that forms sexual gametes (sex cells sperm and eggs in humans). These gametes can then unite to form a zygote.

•reduces chromosome number from diploid to haploid•is where crossing over occurs

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MeiosisOccurs in two major divisions:

• Meiosis I• Meiosis II

Each of these divisions occurs in 4 stages:• Prophase• Metaphase• Anaphase• Telophase

Introduction

• Meiosis is a special type of cell division that occurs in the gonads.

• Its purpose is to create reproductive cells known as gametes.

• These gametes are haploid as they only carry one copy of each type of chromosome.

Introduction• Like mitosis, the gonad cells go through Interphase

prior to meiosis.

• This means that the chromosome number prior to meiosis doubles.

• As with mitosis, meiosis only deals with the separation of the nuclear material, whereas cytokinesis is still needed to split up the organelles and cytoplasm.

Introduction• Unlike mitosis, meiosis

produces 4 non-identical haploid cells.

• Their purpose is for reproduction only.

• Also meiosis has the same stages as mitosis, however they occur twice in order to reduce the chromosome number from diploid to haploid (reduction division).

The first half of meiosis

(Reduction Division)

MEIOSIS IMEIOSIS I

Prophase I

• Nuclear membrane dissolves

• Chromosomes are attached to their copy by a centromere.

• Centrioles appear and move to the poles.

• All this is similar to mitosis.

Prophase I

• All the chromosomes of the same chromosome number (i.e. all chromosome #1 and their copies), known as homologous chromosomes, join together to form a tetrad or bundle of four chromosomes (2 from each parent).

• This is different from mitosis!

Prophase I

• During the tetrad formation, non-sister chromatids can exchange genes to create unique allele combinations that are not seen in the parent.

• This is called crossing over and is one of the two reasons for the uniqueness of the offspring.

Crossing Over

Metaphase I• The tetrads align themselves at

the equator with the maternal pair of chromosomes facing one pole and the paternal chromosome pair facing the other pole.

• All tetrads align independent of one another.

• This is called independent assortment and is the other reason for the variation seen in offspring.

Metaphase I

• Once aligned, the tetrads are held in place by spindle fibres released from the centrioles at the poles.

• The spindle fibres attach to the centromeres of the sister chromatids.

Anaphase I

• The spindle fibres contract and pull apart the tetrad such that one pair of sister chromatids goes to one pole and the other pair goes to the other pole.

• Notice that the centromere did not split apart during this phase. It still holds the chromosome copies (sister chromatids) together.

Telophase I and Cytokinesis

• A nuclear membrane reforms around each new nucleus and the cytoplasm is divided by cytokinesis creating 2 non-identical cells.

• These two cells immediately proceed into the next round of meiosis as there is no second round of interphase.

• In some cell types, telophase I does not occur and the cell proceeds directly to the second round of meiosis.

The second half of meiosis

Meiosis II• The stages of meiosis II

are all exactly identical to the happenings from mitosis.

• The only difference is that they are occurring in two cells at the same time.

• The end product is four, non-identical, haploid cells.

• The process of meiosis occurs somewhat differently in males and females.

Prophase II

• The nuclear membrane dissolves.

• The chromosomes become more visible and are still attached to their copy by the centromere.

• Centrioles move towards the “poles” of the cell.

Metaphase II

• The sister chromatids line up at the “equator” of the cell.

• Spindle fibres emerge from the centrioles and attach to the centromere of each chromosome pair.

Anaphase II• The spindle fibres contract,

breaking the centromere, and pull apart the sister chromatids.

• One copy of the chromosome pair goes to one pole, while the other copy goes to the other pole.

Telophase II

• The last stage of meiosis, where the nuclear membrane starts to reform around the chromosomes that have been pulled to each pole and two new nuclei have formed in each cell.

• The cell membrane starts to pinch inward at the equator signaling the end of meiosis and the beginning of cytokinesis.

• The end product is four, non-identical, haploid cells.

Homework

• Page 174 #1, 2, 10 and the questions on the back of your diagram sheet.

Meiosis Video

http://highered.mcgraw-hill.com/sites/dl/free/0072495855/291136/meiosis.swf

GAMETE GAMETE FORMATIONFORMATION

Gametogenesis: the production of gametes (sex cells) through the process of meiosis.

GAMETOGENESIS

• Meiosis in males

• occurs in the testes• Starts with a

diploid cell called a spermatogonium

• produces four non-identical haploid sperm cells

SPERMATOGENESIS

SPERMATOGENESIS• Occurs all the time from puberty until death. • It takes approximately 68-74 hours for a sperm to be

created.• Meiosis produces approximately 250 000 000 sperm

every day in males!

SPERMATOGENESIS

• Meiosis in females

• Occurs in the ovaries and oviducts

• Starts with a diploid cell called an oogonium.

• Egg production starts before a female is born, but pauses in Meiosis I before the cells (primary oocyte) divide.

• The meiotic process resumes at puberty with ovulation (and fertilization), for 1 cell every month.

OOGENESIS

OOGENESIS• After Telophase I and II only one of the cells receives the

majority of the cytoplasm, resulting in one egg cell and three polar bodies.

• The purpose of the unequal division is to allow the egg cell to have sufficient nutrients to support a zygote immediately after fertilization.