BIO 10 Lecture 9 REPRODUCTION: MITOSIS AND MEIOSIS

BIO 10
Lecture 9
REPRODUCTION:
MITOSIS AND MEIOSIS
Life and DNA
• Living organisms are “survival
machines” for DNA molecules
• DNA molecules carry the information
that enables host cells to protect and
perpetuate them
•
•
•
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Enclose and protect them from “other”
Gather and use energy to fight entropy
Reproduce
Evolve
• A Genome =
complete collection of
an organism’s genetic
information as linked
genes in one or more
long strands of DNA
– Genes provide
information for building
proteins
– Information for making
proteins is stored
digitally in the
sequence of chemical
bases A, C, G, and T in
the double helix.
• In order to perpetuate this
information, each human
cell must be able to
replicate its DNA
• Then divide the DNA
equally into daughter cells
– In eukaryotes, this process is
called mitosis
• Mitosis occurs before the
cytoplasm of the cell splits
in two (cytokinesis), giving
each “daughter cell” one
copy of the original parent
DNA molecule and half the
cytoplasm of the parent cell
DNA Replication
Chromosomes
• DNA is packaged in chromosomes
– Each DNA molecule is very long (~0.02 m) compared to
its width (0.000000002 m)
• Ten million times longer than it is wide
– Compare to a piece of sewing thread (1 mm wide)
• Would stretch 10,000 m = 6.21 miles!
– Nucleus is only ~10 um in diameter but must house ~1
m of DNA (100,000 times longer than the nucleus is
wide)
• Like having a softball stuffed with 6.21 miles of sewing thread
• DNA must be packaged around proteins to keep it
organized = chromatin
• Like storing sewing thread on spools
• The human genome carries about
25,000 genes spread out over 24
different types of chromosomes
– 22 autosomes (# 1-22)
– 2 sex chromosomes: X and Y
– Each chromosome carries its own genes in a
linear order along its length
• Many genomes are smaller than the
human genome, some are larger
– E. coli genome: 1,000 x smaller than humans
– Amoeba Chaos chaos: 200 times larger than
humans!
http://www.genomesize.com/statistics.php
• Because DNA is packaged into chromosomes,
when it divides, the chromosomes also divide
– A replicated chromosome is comprised of two daughter
DNA molecules held together
– The daughter DNA molecules are called “sister
chromatids”
• Many eukaryotes carry two copies of
each chromosome in the genome
– Protects against the loss of a single gene
(and its protein) due to mutation
– Like having a “back-up” system
– Pairs of like chromosomes are called
homologous chromosomes
• Analogy—ask someone how many shoes they
have, and they will tell you how many different
shoes, like 10 pairs. But if you counted the total,
there are actually 20 shoes. Homologues are not
identical; they are variants, like a left and right
shoe
– Humans have 2 sets of 23 chromosomes
each
• Sex chromosomes: humans and other mammals
use chromosomes to distinguish between the
sexes.
– Males have one X and one Y
– Females have two Xs.
• Autosomes: All other chromosomes.
• Humans have 22 different autosomes (numbered
1-22) and two different sex chromosomes (X and
Y)
• Karyotypes are ordered, pictorial arrangements of
chromosomes
The Cell Cycle
• The eukaryotic cell cycle is made up of a
repeating pattern of growth, DNA
replication, mitosis, and cytokinesis.
– Typical animal cell cycle lasts about 24
hours.
• Two main phases:
– Interphase (about 23 hrs, 30 min)
• Cell growth, productive activity
• DNA replication
– Mitosis and Cytokinesis (about 30 min)
• Chromosomes segregated to daughter cells
• Cell divides in two
MITOSIS
• Process by which the cell’s newly duplicated
chromosomes (pair of sister chromatids still
attached) condense, align themselves, and
separate evenly prior to cell division
• Used for growth, replacement of damaged/
dead cells or (in some organisms) asexual
reproduction
• Occurs in a series of four stages:
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–
–
–
Prophase
Metaphase
Anaphase
Telophase
Variations on the Theme
• Plant cells—everything is similar except for
cytokinesis, because plant cells have to break
down and reform the cell wall.
– Vesicles fuse near the metaphase plate to form a
cell plate that grows outward to form a cell wall.
• Prokaryotes (no nucleus)—binary fission.
– Single circular chromosome
– DNA replicates and daughter “rings” unlink
– Simpler than mitosis
Meiosis: Making Sex Cells
MITOSIS
MEIOSIS
• Purpose:
– Asexual reproduction in
single-celled eukaryotes
– Growth and replacement
of dead cells in multicellular eukaryotes
• Process:
– Clones the original parent
cell
– Daughter cells genetically
identical to parent cell
– One cell division
• Location:
– All tissues
• Outcome:
– 2 identical daughter cells
• Purpose:
– Creation of eggs and
sperm for sexual
reproduction
• Process:
– Number of chromosome
sets halved
– Daughter cells contain
half the DNA of the
parent cell and the DNA is
“mixed up”
– 2 cell divisions
- Location:
- Sexual organs only
- Outcome:
- 4 genetically different
eggs or sperm
Why Sex?
• Not found in prokaryotes
– Asexual reproduction only
• Often an option for single-celled
eukaryotes (e.g. yeast)
– May switch from asexual to sexual
reproduction when the environment
changes
• Usually a requirement for multi-celled
eukaryotes
– Most are absolutely dependent on sexual
reproduction
• Advantage of Sex
– Introduces new combinations of heritable
traits in the offspring
– Critically important for large, multi-cellular
organisms that have limited numbers of
offspring and evolve slowly
– Must keep up with the rapid evolution of
parasites, bacteria, and viruses
• These organisms can evolve very quickly
because they have enormous numbers of
offspring
• Asexual reproduction has the advantage of
being very rapid
Halving the number of
Chromosome Sets
• Although superficially meiosis looks
similar to mitosis, it is really very
different
• In addition to creating new
combinations of heritable traits in the
eggs and sperm, meiosis halves the
number of chromosome sets in the
eggs and sperm
• If combining traits were accomplished just by
combining two cells, life would get incredibly
complicated.
– Analogy to couples hyphenating their names
when married.
• If n = number of different chromosomes.
– Human cells are diploid = 2 of each chromosome
(homologues) 2n.
– If you took a somatic cell from a woman (say a bit
of skin) and a man and combined them, you
would have 46 + 46 = 92 (4n) chromosomes. The
next generation would repeat this to 184 (8n), and
so on.
• After only 30 generations, a billion chromosomes per
cell!!
• Must have a way of reducing the
chromosome number by half, to get
23(n) + 23(n) = 46 (2n) total for the new
organism each generation
– Cells with half the number of
chromosomes (one of each) are haploid
and are called gametes (sperm and eggs)
– These cells are made only in sexually
reproducing organisms and only in a
special organ called a gonad (testis and
ovary)
Steps in Meiosis
• Meiosis begins like mitosis does, with
replication of DNA in S phase of interphase
– Involves two cell divisions instead of just one
– Meiosis I and Meiosis II
• Meiosis I
– When homologues separate
– Chromosome number halved
– Genetic variation created by “scrambling up”
chromosomes through two processes:
• Crossing over of homologous pairs during prophase of MI
• Independent assortment of homologue pairs during
metaphase of MI
In Meiosis I, homologous pairs line up,
exchange parts, and “dance together” ...
• Meiosis II
– Is a haploid mitotic division
– Is when sister chromatids separate
Role of Meiosis in Sex Ratio
• Humans have 22 pairs of autosomes and
one pair of sex chromosomes; females
have two X chromosomes, but males
have one X and one Y.
• Sex chromosomes pair like homologues
and separate in meiosis I, so each haploid
cell that results has either an X or a Y in
males.
– The sex of a child is therefore determined by
the sperm at fertilization
Spermatogenesis in Humans
• Spermatogonia: diploid cells that can
divide by mitosis to make more of
themselves, or make cells destined to
become sperm called primary
spermatocytes.
– Primary spermatocytes complete meiosis I to
give rise to two secondary spermatocytes.
– Secondary spermatocytes complete meiosis II
to make four total haploid spermatids.
– Spermatids are matured in about three weeks
to form sperm with flagella tails, concentrated
mitochondria, and a haploid nucleus.
– About 250 million sperm are made each day,
about same number in one ejaculate.
Oogenesis in Humans
• Oogonia: diploid cells that divide by
mitosis early in embryogenesis, never
divide after birth. Most die before birth,
but the remaining oogonia make cells
destined to become eggs, called
primary oocytes.
– Primary oocytes begin meiosis I in the
embryo, but do not complete it until
ovulation to give rise to one secondary
oocyte and a polar body.
• Secondary oocytes complete meiosis II
after being fertilized by a sperm to make
one haploid egg. The other products of
meiosis are polar bodies.
– Oocyte cytokinesis is unequal to ensure that one
large cell, 200,000 times bigger than the sperm,
has enough materials to drive early divisions and
feed the rapidly dividing embryonic cells.
– All mitochondria inherited by the baby are from the
mother’s egg
– Usually only one secondary oocyte is released per
28-day cycle in an adult woman.
Short Review of Lecture 9
• How does DNA carry genetic information? What
is a chromosome? What is a gene?
• What is a genome?
• What is the cell cycle? What happens during
interphase? Mitosis?
• Compare mitosis and meiosis. How are these
processes similar? How are they different?
• What types of organisms under mitosis?
Meiosis?
• What is the advantage of sex? Why are most
complex eukaryotes dependent on sex for the
perpetuation of their genes?
• How is sex determined in humans?