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Meiosis and Sexual Reproduction
Chapter 10
Number of Chromosomes
As sexual organisms, humans receive a set of chromosomes from each parent
Humans have 46 chromosomes, two copies of each chromosome (23 from mom, 23 from dad)
A cell that contains one copy of each chromosome is called haploid
A cell that contains two copies of each chromosome is called diploid
In diploid cells, the two identical chromosomes are called homologous chromosomes
Number of Chromosomes
Homologous chromosomes contain the same genes, but not necessarily the same traits ◦ Ex: Chromosome #6 contains a gene for eye color, but one chromosome may have a blue-eyed trait while the other may have a brown-eyed trait
After S phase, when each chromosome is copied in preparation for division, each cell will then have four copies of each chromosome
Meiosis
During sexual reproduction, each parent will donate a gamete to the offspring ◦ Gametes are reproductive cells (sperm and egg)
◦ When gametes fuse, the cell is called a zygote.
Gametes are haploid. They only contain one copy of each chromosome
The zygote is diploid
Meiosis is the system that produces a haploid gamete for sexual reproduction
Meiosis
Mitosis: ◦ The daughter cells are exact copies of the parent
◦ There are only two cells produced
◦ The daughter cells are diploid
In Meiosis ◦ The daughter cells contain a full set of chromosomes, but which traits each one receives are random mixtures of their parent’s traits
◦ Four daughter cells are produced
◦ The daughter cells are haploid
Meiosis
For gametes, interphase occurs as it does in mitosis
Meiosis then goes through two series of steps, each similar to mitosis ◦ Meiosis I: the cells are reduced from diploid to haploid
◦ Meiosis 2: the chromatids on each individual chromosome are separated
Meiosis only occurs in reproductive organs, while mitosis occurs in all other cells
Meiosis I Prophase I
◦ The nuclear envelope disappears, and spindle reappears
◦ The chromosomes also appear
◦ The homologous chromosomes undergo a process called crossing over.
During prophase I, homologous chromosomes line up next to each other, matching gene for gene
They then trade sections of each other’s chromosomes at random locations.
After crossing over, each chromosome contains parts of it’s original strand of DNA and part of it’s homologue.
Thus, each chromatid is a unique sequence of DNA
Meiosis I
Metaphase I ◦ Each pair of homologous chromosomes will line up next to each other on the metaphase plate
◦ The Law of Independent Assortment: when chromosomes line up in metaphase I, they do not line up in any particular order or on any particular side.
Each chromosome has an equal chance of being placed on the same side with any other chromosome in the cell
◦ When the cell undergoes the first division, it ensures each cell will have at least one copy of each chromosome
Meiosis I
Anaphase I ◦ During Anaphase 1, the spindle fibers begin pulling each homologous chromosome toward each centriole
◦ *Unlike in mitosis, each chromosome still contains two chromatids at this point*
Meiosis I
Telophase I ◦ In telophase I, a small plasma membrane is built between the two cells, but cytokinesis usually does not occur
◦ The nuclear envelope reforms briefly, then disappears again
◦ There are now two separate copies of non-identical chromosomes in two different cells
Meiosis II (Similar to Mitosis)
• Prophase II –Chromosomes appear and nuclear envelopes disappear again
• Metaphase II –Chromosomes line up at the center of the cell at the metaphase plate
• Anaphase II –Individual chromatids separate and move toward separate centrioles
• Telophase II –Cell parts reform, and cytokinesis occurs
–Four new daughter cells have been produced
Human reproductive cycles
When the zygote is formed, it is a combination of the two sets of DNA in each gamete
Males undergo spermatogenesis, which occurs in the testes and produces 4 sperm
Females undergo oogenesis, which occurs in the ovaries and produces 1 egg ◦ An egg is 20 times larger than a sperm cell. ◦ The other three eggs that aren’t used are broken
up and their parts go to make the larger egg.
Most of these will never be used for sexual reproduction, but are produced anyway
It is completely random which genes will be in which gamete during the moment of fertilization
Why all the mixing?
• Crossing over, independent assortment, and random fertilization all exist to ensure gametes are random samples of parental DNA
• Why is this good? –1) Certain genes may be good for one environment, but what if the environment changes? • Only the same genes will be passed on to the next organism
• If the genes are now unsuccessful, its dangerous for the species
–2) Shuffling of genes allow for unique combinations of genes to occur
Why all the mixing?
–3) Allows for different genders
• With no mixing, how do you decide what gender offspring will be?
–4) The organisms with the best traits will most likely survive and, thus, will most likely reproduce
• With random shuffling, the best genes will also be most likely to pass on over multiple generations
–5) Variety of organisms
• A single set of human chromosomes, if randomly fertilized, allowed to go through independent assortment, and allowed to cross over only once will result in approximately how many genetically different human zygotes?
4,951,760,200,000,000,000,000,000,000
Mitosis vs Meiosis
You should be able to answer the following for each stage: ◦ Number of daughter cells
◦ Haploid or diploid
◦ Identical or non-identical
◦ How to identify each stage by name, picture, or description
◦ Examples of cells/organisms that undergo mitosis and meiosis.
Changes in Chromosome number
Chromosomal mutations occur when the gametes formed during meiosis contain unusual numbers of chromosomes
After meiosis I, you should have two cells, each with two copies of each chromosome
After meiosis II, you should have four cells, each with one copy of each chromosome
It’s always possible that homologous chromosomes fail to separate or sister chromatids fail to break
Polyploidy
Polyploidy is a chromosomal mutation where a cell has more than two copies of each chromosome ◦ Triploid = 3n
◦ Tetraploids = 4n
◦ Pentaploids = 5n
Polyploidy occurs often in plants, and is one reason for huge evolutionary diversity of flower designs and plant species
*A set of gametes may go through meiosis and form polyploids, but only 1 out of these 4 gametes will be fertilized.*
Monosomy and Trisomy
If an organism has one less or one more of a specific chromosome, it is referred to as monosomy or trisomy
Monosomy and Trisomy is a result of nondisjunction ◦ Both members of a homologous chromosome enter the same daughter cell during meiosis I
◦ Both sister chromatids fail to separate in meiosis II
Down syndrome is the most common human trisomy, when the individual has a third #21 chromosome.
Sex Chromosome number It is possible to inherit an abnormal number of
sex-chromosomes as well. ◦ XO—Turner syndrome, female
Short, broad chested and extra folds of neck skin.
Do not undergo puberty, menstruation, but are capable of giving birth (thanks to modern infertility treatments) and can lead fairly normal lives
◦ XXY—Klinefelter syndrome, male The male sexual organs are underdeveloped, but the
female organs (breasts, in particular,) are slightly overdeveloped
Very large hands, feet and arms
◦ XXX—Poly-X female Usually, poly-x females lead normal lives. Only slight
physical abnormalities are evident
◦ XYY—Jacob’s syndrome, male Taller than average, persistent acne, and communication
problems
Chromosome Structure
Some chromosomes are more fragile than others, and damage is always possible
Most of the time damage is repaired by the cell. If not, it can result in more chromosome mutations
Deletion ◦ In a deletion, a chromosome breaks, usually at the end
of the chromosome ◦ Williams syndrome is a loss of the end of #7
chromosome Williams children are academically and physically awkward,
but show excellent communication and musical skills ◦ Cru de Chats syndrome is a loss of the #5 chromosome
end Individuals are mentally retarded with facial abnormalities,
and an enlarged larynx results in an infant cry similar to a cat’s
Chromosome Structure
A translocation is when a chromosome section moves from one chromosome to another ◦ Down syndrome (Trisomy 21) is a result of a section of the #21 chromosome breaking and attaching to a #14 chromosome, resulting in three #21 chromosomes
◦ Alagille syndrome, when a translocation occurs between #20 and #2 chromosomes
◦ The missing section of chromosome #20 results in severe itching and irritation of eyes, skin, and genital areas
Chromosome Structure
Other types of chromosome break mutations:
Duplication ◦ A chromosome section breaks and reattaches to it’s sister chromatid, resulting in a repeated section
Inversion ◦ A chromosome section breaks and reattaches, but backwards
Extra Credit Mitosis Question
This question is worth an extra 5% on your essay exams
You may check your answers with me ahead of time for a yes or no response as many times as you like. Turn in your answer to the box by Friday, the 15th.
At the beginning of mitosis, there is one cell. At
the end of mitosis there are two cells. These two cells are identical in every way to their
parent cell. Absolutely no construction, synthesis, diffusion or
osmosis occurs during mitosis. Where does the cell get the materials for building a
2nd identical cell if it can’t build or accept any materials during mitosis?
