Meiosis and Sexual Reproduction

It’s all about chromosomes What are chromosomes? A structure composed of DNA, that contains the genetic material

There are ________ chromosomes in a human body cell Sperm cells and egg cells[also called gametes] have half the normal number of chromosomes The sperm cell has ________ chromosomes The egg cell has ________ chromosomes

Remember, each gamete only has half the normal number of chromosomes a cell that has half the normal number is called haploid [gametes; egg cells] A cell that has double the amount of the haploid cell is called diploid [body cell]

The human haploid number is _____ The human diploid number is _____ The diploid number of a fire breathing yeti is 50, what’s the haploid number? the haploid number of a flying Liger is 74, what’s the diploid number?

Each diploid cell has a set of homologous chromosomes Homologous chromosomes: A pair of chromosomes that contain the same gene sequences, one of the pair is derived from the mother and the other from the father.

In order to examine chromosomes, they are prepared as a karyotype A karyotype is a display of chromosomes arranged in homologous pairs

How are Karyotypes prepared? They are prepared from isolated somatic cells, which are treated with a drug to stimulate mitosis and then grown in a culture for several days Cells arrested in metaphase, are stained and then viewed with a microscope equipped with a digital camera

An image of the chromosomes is displayed on a computer monitor Digital software is used to arrange them in homologous pairs

Application Technique Pair of homologous duplicated chromosomes Centromere 5 µm Sister chromatids Metaphase chromosome Figure 13.3 Research method: preparing a karyotype

Why is it important to study the karyotype of chromosomes?

Screen for defective chromosomes Karyotyping can be used to: Screen for defective chromosomes Screen for abnormal numbers of chromosomes See the gender

gametes The sperm and egg cells are called gametes The forming of gametes is called gametogenesis

The formation of sperm cells is called spermatogenesis The formation of eggs cells is called oogenesis

Your body has special cells in the reproductive organs that are called germ cells. These cells are the only cells that can undergo meiosis and become gametes such as sperm and ova.

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Meiosis A process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.

Meiosis stages: Meiosis 1 It’s the first cell division in this process It’s called reduction division During this process, the chromosome number is halved

Interphase It’s the same interphase as mitosis which consist of 3 parts…. What are they?

Prophase 1 the nucleus membrane disintegrates The centrioles move to opposite sides of the cell while forming spindle fibers

The homologous chromosomes pair up in a process called synapsis: Synapsis is the pairing of homologous chromosomes, one from each parent,  during early meiosis

during synapsis, a process called crossing-over occurs Crossing-over: the process where homologous chromosomes exchange different segments of their genetic material

Crossing over creates new combinations of genes in the gametes that are not found in either parent, contributing to genetic variation.

Prophase 1

Metaphase 1 Now the chromosomes line up down the middle of the cell This time not in a single file, they’re lined up in homologous pairs

Metaphase 1

Anaphase 1 The spindle fibers pull the homologous chromosomes away from each other

Anaphase 1

Telophase 1 2 Nuclei form The cell “pinches in”

Cytokinesis 1 The cell separates into 2 cells, each with 23 chromosomes

End of meiosis 1 Meiosis 1 results in two haploid cells. Each cell has half the number of chromosomes as the original cell[23 chromosome each].

Meiosis 2 Also passes through 4 stages The chromosome number is not being reduced Now the chromatids from each double-stranded chromosomes are being separated

Prophase 2

Metaphase 2 The chromosomes line up down the middle of the cell.

Anaphase 2 Sister chromatids separate and move to opposite ends of the cell.

Telophase 2 Four nuclei form around chromosomes

Cytokinesis 2 The cytoplasm splits The cells divide into 4 haploid cells

Gamete Formation In males, meiosis results in 4 sperm cells In females, meiosis results in 1 egg cell and three polar bodies, which are not used in reproduction.

What is the purpose of polar bodies during oogenesis?  they ensure that the ovum will have most of the cytoplasm. They rid the body of defective sets of chromosomes, leaving the "good" set within the ovum. Then they dissolve

The sexual life cycle process involved in human reproduction is: Meiosis Fertilization Mitosis of the zygote differentiation The sexual life cycle process involved in human reproduction is:

Mitosis vs Meiosis mitosis meiosis 1 division occurs during mitosis 2 sets of divisions occur during meiosis DNA replication occurs during interphase DNA replication occurs once before meiosis 1 Synapsis of homologous chromosomes does not occur Synapsis of homologous chromosomes occurs during prophase 1 2 identical cells are formed per cell cycle 4 haploid cells(n) are formed per cell cycle 2 Daughter cells are genetically identical The daughter cells are not genetically identical because of crossing over Mitosis occurs only in body cells Meiosis occurs in reproductive cells Mitosis is involved in growth and repair Meiosis is involved in the production of gametes and providing genetic variation in organisms

Why is meiosis important? It maintains the normal species chromosome number by making haploid gametes It increases genetic variability by recombining genes in eggs and sperm

Terms 1. crossing-over 2. recombination 3.independent assortment 4. chromosomal abnormalities 5. differentiation

1. Crossing - over When homologous chromosomes exchange genetic pieces during prophase 1 of meiosis

2. recombination When a new combination of genes are made on a chromosome during crossing-over

The results of recombination are: A new combination of inheritable traits Genetic variation

3. Independent assortment Each time a body cell goes through meiosis to make a gamete, all the chromosomes line up a different way during the metaphase part The number of different ways they line up is independent assortment

4. Chromosomal abnormalities When there’s an incorrect number of chromosomes in a gamete Can cause birth defects or the zygote fails to develop

5. differentiation The process by which cells in an embryo develop a specialized form and function It occurs during the 5th week of pregnancy