The nucleus with chromatin The nucleus just before cell division (contains chromosomes)

Karyotype Photomicrograph of the chromosomes in a dividing cell. A picture of your chromosomes! Challenge question: Name a phase of the cell cycle that the cell must be in in order to get a picture of chromosomes. GOA ?- Why do you think the cell is chemically stimulated to go into mitosis before geneticists capture the DNA? Because you cannot capture a photomicrograph of chromatin, only chromosomes after they have been coiled and are nice and fat.

In humans e.g. 23 chromosomes in haploid 2n = 46; n = 23 2n = 223 = ~ 8 million possible combinations!

How are karyotypes useful to society? Doctors use karyotyping to find genetic disorders in humans and even some animals. Pregnant women can choose to look at a karyotype of their baby before it is born to see if there are going to be any problems.

Autosomes vs. Sex Chromosomes The chromosomes that determine the sex of the individual Female: XX Male: XY Autosomes A chromosome that is not a sex chromosome What is the sex of this karyotype? Male How many autosomes do we have? Sex chromosomes?

Homologous chromosomes One of a pair of genetically similar chromosomes Two copies of each autosome…why? One from mom, one from dad!

Homologous chromosomes vs. sister chromatids Sister chromatids or Homologous pair? Key here is to make sure students see that it can be tricky to distinguish between sister chromatids and homologous chromosomes. If they see a centromere, then they know this is a single chromosome that has been replicated into chromatids. GOA ?- What is a centromere? The region where the sister chromatids are held together until they separate during cell division.

Homologues Chromosomes exist in homologous pairs in diploid cells. Exception: Sex chromosomes (X, Y). Other chromosomes are known as autosomes, they have homologues.

Why do we need two copies of a chromosome (homologous chromosomes)? So organisms can have sexual reproduction!!! Sex creates variety and diversity in life!!! Bag of candy- one type- always the same  (asexual reproduction) Bag of candy- two types- possibility of variety  (sexual reproduction)

What are those bands on the chromosomes? Dyed patterns Patterns code for genes GENES ARE FOUND ON CHROMOSOMES!

Trisomy- a genetic condition in which the organism has 3 homologous chromosomes instead of 2.

Math problem I’m a combination of Which should give me…. Wrong!! mom (46 chromosomes) and dad (46 chromosomes) Which should give me…. 92 chromosomes! Wrong!! The process called meiosis solves this problem.

Sexual vs. Asexual Reproduction The BIG PICTURE 1 1 Sexual Reproduction 1 ½ 1 Parent offspring egg 1 parent MEIOSIS fertilization offspring ½ parent sperm

Diploid vs. Haploid Diploid cell Haploid cell “di” means two a cell that has a full set of chromosomes, or two copies of each chromosome; all 46 for humans. Somatic cells are diploid Haploid cell Sounds like “half”-ploid A cell that has half the number of chromosomes, or just one copy of each chromosome; 23 for humans sex cells are haploid

Meiosis Making of the gametes: Specialized cell division that results in the creation of the sex cells (egg and sperm). Homologous chromosomes separate from each other so that each new cell only receives one of the pair- or half the number of chromosomes/genes

Who does Meiosis? Only sexually reproducing organisms have cells that go through meiosis. So, do all of your body cells go through meiosis? NO!!! Which cells go through meiosis then?

Gonads make gametes! Testes make sperm

Gonads make gametes! Ovaries make eggs

Meiosis in the Gonads

Vocab terms Somatic cells Sex cells (gametes) “soma”: greek word for body Somatic cells are the cells that make up the body of an organism Sex cells (gametes) Sperm and egg; the cells that are made by meiosis Have half the number of chromosomes as somatic cells.

Sexual vs. Asexual Asexual reproduction Sexual Reproduction 1 ½ 1 ½ 1 1 Sexual Reproduction 1 ½ 1 One parent Identical offspring LIKE mitosis Parent offspring egg Two parents Similar but not identical offspring Meiosis parent sperm fertilization ½ 1 offspring

Why is Meiosis necessary? Sperm To reduce chromosome number and increase variety In sex cells- 1n sperm +1n egg= 2n baby N egg N 2N baby

Haploid vs. Diploid Diploid (2n) Haploid (1n) Cell that has two sets of chromosomes. Somatic cells Haploid (1n) Cell that contains only one set of chromosomes Sperm and Egg 1n + 1n = 2n Why is it important that the egg and sperm cells be haploid instead of diploid?

Diploid and Haploid Organism 2n 1n Human 46 23 Horse 64 32 Cat 16 Carrot Earthworm 36 18 Sand Dollar 52 26 Chimpanzee 48 24 Gorilla

Checking for Understanding Compared to the parent cell, how many chromosomes do the daughter cells have after meiosis? ½ So if human somatic cells have 46 chromosomes, how many chromosomes do the sex cells have? 23 In asexually reproducing organisms, how many chromosomes does the offspring have compared to the parent? The same Would the cells that make up the leaf of a plant be considered somatic or sex cells? Somatic Is a sperm cell diploid or haploid? haploid

Stages in Meiosis: Interphase Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II Full set of chromosomes (2N) DNA has doubled (4N) Homologous chromosomes line up Homologous chromosomes separate 1st cell division (2N) DNA does not double 2nd division Sperm & Egg (1N) 2nd division (1N)

Meiosis 1 First division of meiosis Prophase 1: Each chromosome dupicates and remains closely associated. These are called sister chromatids. Crossing-over can occur during the latter part of this stage. Metaphase 1: Homologous chromosomes align at the equatorial plate. Anaphase 1: Homologous pairs separate with sister chromatids remaining together. Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.

Independent assortment

Independent assortment Number of combinations: 2n e.g. 2 chromosomes in haploid 2n = 4; n = 2 2n = 22 = 4 possible combinations

Crossing over Chiasmata – sites of crossing over, occur in synapsis. Exchange of genetic material between non-sister chromatids. Crossing over produces recombinant chromosomes.

Meiosis II Second division of meiosis: Gamete formation Prophase 2: DNA does not replicate. Metaphase 2: Chromosomes align at the equatorial plate. Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole. Telophase 2: Cell division is complete. Four haploid daughter cells are obtained.

Mitosis vs. meiosis

Meiosis – key differences from mitosis Meiosis reduces the number of chromosomes by half. Daughter cells differ from parent, and each other. Meiosis involves two divisions, Mitosis only one. Meiosis I involves: Synapsis – homologous chromosomes pair up. Chiasmata form (crossing over of non-sister chromatids). In Metaphase I, homologous pairs line up at metaphase plate. In Anaphase I, sister chromatids do NOT separate. Overall, separation of homologous pairs of chromosomes, rather than sister chromatids of individual chromosome.

Meiosis creates genetic variation During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n) Meiosis results in genetic variation by shuffling of maternal and paternal chromosomes and crossing over. No daughter cells formed during meiosis are genetically identical to either mother or father During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring.

Random fertilization Independent assortment creates 8 million possible combinations for gametes, plus in meiosis I crossing over occurs At least 8 million combinations from Mom, and another 8 million from Dad … >64 trillion combinations for a diploid zygote!!!