Cell Reproduction (continued)

Review

• Prokaryote and eukaryote cells reproduce differently.

• Prokaryotes reproduce by binary fission (they split in two), which is an asexual process.

• Eukaryotes reproduce sexually and asexually.

• Somatic cells in eukaryotes reproduce asexually by mitosis.

Mitosis: A few more facts

• At the end of mitosis, you end up with two identical daughter cells.

• Each daughter cell (in a human) has exactly 46 chromosomes.

• If a human cell has 46 chromosomes, it is diploid (2n), where n = 23.

• ALL somatic cells are diploid. They contain 2n chromosomes.

Now, let’s talk about sex…

• In Biology, when you talk about sex, you’re talking about meiosis.

• The reproductive cells, or gametes, of eukaryotes reproduce sexually by a process called meiosis.

• Meiosis is very similar to mitosis EXCEPT:– Meiosis consists of 2 division cycles– Meiosis starts with pairs of homologous chromosomes (or a

tetrad of chromatids)– You end up with 4 daughter cells (sperm or egg)– Each daughter cell is genetically different– Each daughter cell is haploid (n), which means it contains half

as many chromosomes as somatic cells.

Diploid vs. Haploid

• A diploid cell has a full set of chromosomes. It is 2n. – 1 set from Mom (1n)– 1 set from Dad (1n)

• A cell with a half set of chromosomes is called a haploid cell. It is 1n.

• Two gametes (egg and sperm) combine to form a diploid zygote.

Recall the structure of a chromosome:

chromatin

DNA

chromosome

Homologous Chromosomes• Because we get 1 set of

chromosomes from mom, and 1 from dad, we have 2 versions of the same chromosome (homologs).

• These 2 chromosomes have genes in the same places (called loci).

• However, while Mom’s version of the gene might code for blue eyes, Dad’s may code for brown.

• Different versions (here represented by colors) of genes are called alleles.

The Sex Chromosomes

• One pair of chromosomes is unique. These are the sex chromosomes (X & Y chromosomes). They determine whether or not the baby will be a boy or a girl.

• Girls have 2 X chromosomes.

• Boys have 1 X and 1 Y chromosome.

Chromosomes & Karyotyping

• Your other 22 pairs of chromosomes are called autosomes.

• They contain the genes that code for everything else. (see Human Genome Project Poster)

• A karyotype shows all 23 pairs of chromosomes. It can show whether or not a person contains certain chromosomal abnormalities or conditions.

Some Chromosomal Abnormalities/Conditions

• Irregular number of chromosomes- results in fetal death, or severe retardation, malformation, & short life span.– Trisomy (3 copies)– Monosomy (1 copy)

• Ex.: Down Syndrome- results from extra copy of chromosome 21

Some Chromosomal Abnormalities/Conditions

• Female Sex abnormality- Turner syndrome- can occur when there is only one copy of the X chromosome.

• Symptoms include webbed neck, widely- spaced under-developed breasts, and short stature.

Some Chromosomal Abnormalities/Conditions

• Both of the previous disorders were a result of mechanical errors that occurred during meiosis. They are called nondisjunction errors.

• Sometimes chromosomal abnormalities only show up in some cells. This is called mosaicism. In these cases, the abnormality will usually be less severe.

• http://www.biology.arizona.edu/human_bio/activities/karyotyping/karyotyping.html

Back to Meiosis: Why is sex good?

• Organisms reproduce sexually because it provides variation in the species. With the exception of identical twins, every single one of us is different, and different is good, especially when differences give us an advantage.

Meiosis I & II• The first division cycle of meiosis is called

Meiosis I. It consists of:– Prophase I– Metaphase I– Anaphase I– Telophase I

• The second division cycle of meiosis is called Meiosis II. It consists of: – Prophase II– Metaphase II– Anaphase II– Telophase II

Prophase I• This is the first stage of

Meiosis I. • The cell has made extra

chromosomes, and now contains 4n (humans = 92).

• The nuclear membrane disappears.

• Homologous chromosomes (4 chromatids) pair up. This is called synapsis.

• Sometimes, they cross legs and switch parts. This is called crossing over.

Metaphase I

• Homologous chromosomes line up in the middle of the cell, attached to the spindle.

• These chromosomes line up randomly behind other pairs. This is called independent assortment.

(Metaphase in the 2nd round of Meiosis)

Anaphase I

• Homologous chromosomes separate, and move to opposite ends of the cell. Note that they still have centromeres.

Telophase I

• Just like in telophase of mitosis, the nucleus reforms around the chromosomes at each end of the cell, and the cells begin to pinch in and separate by cytokinesis.

• The chromosomes unwind to chromatin

Prophase II

• Chromatin condenses again, to reform chromosomes.

• These chromosomes only have 2 chromatids, just like prophase in mitosis.

• The nuclear membrane disappears and spindle forms.

Metaphase II

• The chromosomes line up in the middle.

• The way that they line up is completely random- once again, this is called independent assortment.

Anaphase II

• The centromeres break, and chromatids move away from each other to opposite ends of the cell.

• How they separate is also random, and is called segregation.

Telophase II

• The nuclear membrane reforms, and chromosomes unwind.

• The cells pinch inward, and cytokinesis occurs.

Daughter Cells: Gametes

• The end result of meiosis is 4 haploid daughter cells.

• Each daughter cell has half the number of chromosomes a normal diploid cell.

Sperm vs. Egg Production

• The process just described is exactly how sperm are produced.

• However, eggs are produced slightly different. Instead of the cytoplasm being evenly split between daughter cells, most of it is donated to one of the cells. The smaller egg cells die.

Why sex? Variation!

• The advantage of sexually reproducing is the variation in offspring that it causes.

• Variation comes from 3 things in meiosis:– Crossing over- when homologous

chromosomes exchange legs– Independent Assortment- how

chromosomes line up behind others randomly– Segregation- how chromosomes separate

into the daughter cells randomly

Animated Meiosis

                                             

http://highered.mcgraw-hill.com/olc/dl/120074/bio17.swf