Chapter 11.4 Meiosis and Sexual Reproduction This picture shows gametes ___________ There are many small sperm cells competing to _____________ a larger egg cell uniting fertilize

Objectives Summarize the events that occur during meiosis. Relate crossing-over, independent assortment, and random fertilization to genetic variation.

11.4 Vocabulary Haploid Diploid Somatic cell Germ cell Gamete Meiosis Homologous chromosome Crossing over Independent assortment Sex chromosomes Autosome

46 diploid 2n sets mother father Body cells are called somatic cells All of your somatic cells contain ____ chromosomes, which means they are _________ cells A diploid cell is mathematically written as ______, which means they have two _______ of chromosomes… you inherit one set from your _____________ and ____________ 46 diploid 2n sets mother father

Sexual Reproduction – Eukaryotic Cells ______ pair of your chromosomes are called your _______ chromosomes, because they are involved with determining your ____________ Your other 22 pairs of chromosomes are called ________________ sex gender autosomes

X Your two sex chromosomes are referred to as _____ and _____ You inherit one from each parent ______ = male ______ = female Females can donate only X’s while males can donate an X or a Y… Do males or females determine the sex of an offspring? Y XY XX males

sex two gametes egg sperm haploid n one Your _________ cells reproduce sexually because ______ cells are needed to make another cell Your sex cells, also called _________________, consist of ________ or _________ cells Gametes are _____________ cells, which are written mathematically as ______, meaning they have a _____________ set of chromosomes two gametes egg sperm haploid n one

23 unite fertilization zygote If human somatic cells (2n) have 46 chromosomes…how many chromosomes do gametes (n) have? When two gametes _____________, they make a diploid body cell in the process of _______________ This fertilized cell is called a ____________ 23 unite fertilization zygote

mitosis adult into an ___________ organism ` The zygote will then continue to grow and develop through ____________ into an ___________ organism mitosis adult

homologous shape size content ` In order for sexual reproduction to occur, organisms must inherit ____________ chromosomes, which are similar in ________, ________, and genetic __________ homologous shape size content

` Your 46 chromosomes are actually ____ sets or pairs of homologous chromosomes 2

Formation of Haploid Cells Meiosis is a form of cell division that __________ the number of chromosomes when forming specialized reproductive cells, such as __________ or _____________ These specialized cells are _______________ Since the chromosome number is being ______________ by half, this process is often called cell __________________ halves gametes spores haploids reduced reduction

Quick Check Review! What are the two types of human gametes? What do gametes do? Why is it important that they contain half the number of chromosomes as found in a diploid cell? Sperm and egg cells They fuse to create a fertilized zygote So that they create a diploid cell when they fuse

Meiosis involves two divisions of the nucleus called… _________________ Before meiosis begins, the DNA of the original cell must be copied during _______________ This original cell is a _________________ cell Meiosis I Meiosis II interphase diploid

Homologous Chromosomes & Sister Chromatids When homologous chromosomes copy themselves before cell division, they take on the characteristic X-shaped appearance with each side of the X being called a sister chromatid Sister chromatids are identical pieces of DNA held together by the centromere

Stages of Meiosis Meiosis I includes…. Meiosis II includes I II I II I Prophase ___ Metaphase ___ Anaphase ___ Telophase ___ Prophase ____ Metaphase ____ Anaphase ____ Telophase ____ I II I II I II

Prophase I Nuclear ______________ and _______________ break down _____________ is arranged __________________ become visible How many chromosomes are in the cell? How many chromatids? membrane nucleolus Spindle Chromosomes 4 8

Prophase I Meiosis This stage differs from mitosis because the homologous chromosomes ________ up in a process called ___________ pair synapsis Mitosis

structure traits mom dad identical tall short Review...what are homologous chromosomes? They are a pair of chromosomes that are similar in ________________ and that also contain genes for the same ____________ You inherit one from ______ and one from ______ They are not ________________ For example…dad’s chromosome may have a _________ gene while mom’s has a __________ gene structure traits mom dad identical tall short

Since they are not identical, the homologous chromosomes are ______________ differently However, the identical copies within each chromosome are colored the same…what are they called? colored sister chromatids

The homologous structures pair up so ____________ that they form a four part structure called a ________ Within a tetrad, a process called ________________ may occur tightly tetrad crossing over

During crossing over, areas on _______________ chromatids may exchange genetic material This picture shows a ___________ cross over nonsister single Letters = genes

Before crossing over, how many different gene combinations existed? After crossing over, how many different gene combinations exist? 2 4

Metaphase I Meiosis The __________ of homologous chromosomes line up along the _______________ This arrangement is also _____________ to meiosis pairs equator unique Mitosis

Anaphase I Homologous _______________ are pulled apart by spindle fibers and moved towards ______________ poles of the cell chromosomes opposite

Telophase I and Cytokinesis Prophase I Opposite of ______________ Cytokinesis splits the __________________ and separates the ______ new cells How many chromosomes are in each new cell? The chromosome number has been reduced by _______ and the cells are now _____________ cytoplasm 2 2 half haploid

After Meiosis I After Mitosis 4 chromosomes per cell 2 chromosomes per cell

VERY IMPORTANT!!!!!!!! These two cells do NOT enter ________________ again and do NOT copy their ________ again! They both proceed with meiosis ____. interphase DNA II

Prophase II In each cell, the nuclear membrane and nucleolus ___________ _____________ is arranged dissolve Spindle

Metaphase II individual Now, _______________ chromosomes line up along the equator in each cell

Anaphase II Sister _____________ are pulled towards the opposite poles of each cell After the chromatids separate, what are they called? chromatids chromosomes

Telophase II and Cytokinesis In each cell, the nuclear membrane and nucleolus ___________ The spindle ______________ Chromosomes will ____________ Cytokinesis splits the cells into ______ unique haploid cells reform dissolves uncoil 4

Meiosis and Genetic Variation Meiosis is an important process that allows for the rapid generation of new genetic ________________. Three mechanisms make key contributions to this genetic variation: _________________________ combinations Independent Assortment Random fertilization Crossing over

Independent Assortment Refers to the _____________ distribution of homologous chromosomes during meoisis I In metaphase 1 the homologous chromosomes may pair and line up randomly on either side random

segregate This affects how the chromosomes ________________ and are passed along to the ________________ gametes

Since humans have 23 pairs of homologous chromosomes…each pair has ____ ways it may face while lining up Mathematically, the total number of possible gametes formed is… 2n or 223 = 8,388,608 gametes 2

Random fertilization fuse offspring variation Each of the possible gametes (223) must _______ with another one of the possible gametes (223) during fertilization Mathematically, the total number of possible zygotes is… 223 x 223 = 70,368,744,000,000 zygotes So many different ________________ with great _________________ may result fuse offspring variation

Crossing over Since homologous chromosomes may __________ genetic information within a tetrad, the number of possible gametes and zygotes is actually ___________________ exchange unlimited

Importance of Genetic Variation Meiosis and the joining of gametes are essential to __________________ The pace of evolution is sped up by genetic _______________. Each offspring contains a combination of genes that is ________________ from each parent evolution recombination different

Objective 1 pair tetrads synapsis Crossing haploid chromatids 4 Summarize the events that occur during meiosis. At the beginning of Meiosis I, homologous chromosomes ________ tightly to form ___________ in a process called _____________. ____________ over may occur. When the homologous chromosomes separate, two ____________ cells are formed During Meiosis II, the sister ______________ separate, forming _____ different gametes. pair tetrads synapsis Crossing haploid chromatids 4

Objective 2 Relate crossing-over, independent assortment, and random fertilization to genetic variation. Crossing over – Independent Assortment – Random fertilization - Changes gene combinations on Non-sister chromatids of homologous chromosomes The homologous chromosomes line up and segregate randomly during Meiosis I. The possible gametes fuse randomly