Meiosis

What are we a combination of?

our mom and dad

• Our characteristics and traits are inherited from our parents

What are the various characteristics among individuals due to?

2 sex cells (eggs and sperm) combining during sexual reproduction

Heredity

the passing on of physical or mental characteristics genetically from one generation to another

Trait

a genetically determined characteristic

• traits are located on chromosomes

Where are traits located?

on chromosomes

Genes

arranged segments of DNA on chromosomes

• genes are what control the production of proteins

• there are hundreds of genes on each chromosome

How many genes are on each chromosome?

hundred

Why do humans have 46 chromosomes in their body cells?

each parent contributes 23 chromosomes (23 chromosomes x two parents = 46 total chromosomes)

Body cells = ?

Body cells = somatic cells

Sex cells = ? = ?

Sex cells = germ cells = gametes

Autosomal chromosomes = ?

Autosomal chromosomes = 22 pairs

Sex chromosomes = ?

Sex chromosomes = 1 pair

Homologous chromosomes

the chromosomes that make up a pair–one from each parent

• They have the same length, same centromere position, and carry genes controlling the same traits

What do homologous chromosomes ALWAYS code for?

the same trait, but they might not code for the same version (allele) of that trait

Gametes

sex cells that have half the number of chromosomes

• sperm and egg

How many chromosomes does each human gamete have?

23 chromosomes

What does the letter “n” represent

the number of chromosomes

Haploid

a single set of chromosomes = n

Diploid

two of each chromosome = 2n

Fertilization

the process by which one haploid gamete combines with another to form a zygote

Before fertilization:

• Egg = n

• Sperm = n

After fertilization:

• Zygote = 2n

Meiosis

The process through which gametes are formed → cell division that reduces the number of chromosomes

• AKA reduction division

Where does meiosis occur?

in the reproductive structures of organisms that reproduce sexually

• testes and ovaries

Diffferences between Meiosis and Mitosis

• Mitosis = 2n chromosomes

• Meiosis = n chromosomes

• Meiosis reduces the chromosome number by involving 2 consecutive cell divisions

• After meiosis, a cell with 2n chromosomes will have n chromosomes

• Meiosis involves 2 cell divisions

Meiosis I

The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated

Interphase: When cell undergoes meiosis

• DNA is loose and stringy (uncoiled) = chromatin

• Duplicate centrosomes, which will regulate the movement of chromosomes during meiosis

• Duplicate the DNA

P-I (prophase 1)

• Chromatin is going to coil and condense into thick, short, stubby bodies

• Nuclear envelope (membrane) disintegrates

• Centrosomes reach opposite poles

• Microtubules from cytoskeleton begin to form a spindle, extending from the centrosomes

• Crossing over occurs

Crossing over

the exchange of genetic material between homologous chromosomes during meiosis

M-I (metaphase 1)

• Homologous chromosomes are lined up in the middle of the cell

• Microtubule attaches to centromeres

A-I (anaphase 1)

• Centromere and homologous chromosomes split in two

• Homologous chromosomes are pulled toward opposite poles and the sister chromatids stay together!

T-I (telephase 1)

• Chromosomes arrive at opposite poles and begin to decondense

• Nuclear envelope reforms

• The mitotic spindle breaks down

• Cleavage furrow forms

Cytokinesis

forms 2 daughter cells

Meiosis II

The second division separates sister chromatids

• these chromatids may not be identical due to crossing over in prophase I

P-II (prophase 2)

• chromosomes condense

• nuclear membrane dissolves

• centrosomes move to opposite poles

M-II (metaphase 2)

Spindle fibers from opposing centrosomes attach to chromosomes (at centromere) and align them along the middle of the cell

A-II (anaphase 2)

Spindle fibers contract and separate the sister chromatids and move to opposite poles

T-II (telophase 2)

• chromosomes decondense

• nuclear membrane reforms

• cleavage furrow forms

Cytokinesis during Meiosis II

cells divide to form four haploid daughter cells

Starting with a cell where 2n = 8:

• The original parent cell has 8 chromosomes

• This cell is unduplicated, so it is diploid

• After Meiosis I there will be 2 cells with 4 chromosomes (2 sets of sister chromatids) in each cell. The cell is now haploid

• After Meiosis II there will be 4 cells with 4 chromosomes in each cell. The cell is haploid.

Down’s Syndrome (Trisomy 21)

Extra copy of chromosome 21

Turner’s Syndrome (XO) -

Affected female is missing an X chromosome. Short, slow growth, heart problems, infertility

Klinefelter’s Syndrome (XXY)

Extra X chromosome in males. Low testosterone levels, underdeveloped muscles, sparse facial hair

What happens most of the time when there is a chromosomal abnormality?

miscarriage of the offspring

ex. of chromosomal abnormalities

• Trisomy 16 is the most frequent one

• Trisomy 22

• Trisomy 13

• Monosomy 1-22 (all except Turner Syndrome)

Why is Meiosis Important?

• It results in gamete variation

• It allows species to reproduce

• It can aid the repair of genetic defects