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Chapter 6 Part I
Meiosis and Sexual Life Cycles
http://www.sumanasinc.com/webcontent/animations/content/meiosis.html
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Overview: Variations on a Theme
• Living organisms are distinguished by their ability to
reproduce their own kind:
•
GENES are hereditary (inherited) units made up of DNA
that code for a particular trait/ characteristic.
•
HEREDITY is the transmission of traits from one
generation to the next.
•
VARIATION is demonstrated by the differences in
appearance that offspring show from parents and
siblings.
• Genetics is the scientific study of heredity
and variation.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Acquiring Genes from Parents
• In a literal sense, children do not inherit particular
physical traits from their parents…it is genes that
are actually inherited.
•
Genes are the units of heredity, and are made up of segments of
DNA
•
Genes are passed to the next generation through reproductive
cells called gametes (sperm and eggs).
•
Each gene has a specific location called a locus on a certain
chromosome.
• Most DNA is packaged into chromosomes.
• One set of chromosomes is inherited from each
parent.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-4
Comparison of Asexual and Sexual Reproduction
• In asexual reproduction, one parent produces
genetically identical offspring by mitosis.
• A clone is a group of genetically identical
individuals from the same parent.
• In sexual reproduction, two parents give rise
to offspring that have unique combinations of
genes inherited from the two parents.
• Advantages  greater genetic variation!
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Sexual Life Cycles
• Fertilization and
meiosis alternate in
sexual life cycles
• A life cycle is the
generation-togeneration
sequence of stages
in the reproductive
history of an
organism
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Sets of Chromosomes in Human Cells
• Human somatic cells (body cells/ any cell other
than a gamete) have 23 pairs of chromosomes.
• A karyotype is an ordered display of the pairs of
chromosomes from a cell .
• The two chromosomes in each pair are called
homologous chromosomes, or homologs.
•
Chromosomes in a homologous pair are the same length and
carry genes controlling the same inherited characters. One is
inherited from the mother, and the other from the father.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
5 µm
Pair of homologous
replicated chromosomes
Centromere
Sister
chromatids
Metaphase
chromosome
Homologous Chromosomes
• HOMOLOGOUS CHROMOSOMES are
chromosome pairs of the same length,
centromere position, staining pattern,
and gene possession for the same
characters:
–
One homologous chromosome is
inherited from the organism’s father and
the other from the mother.
• DIPLOID – means “two sets”
–
This represents the cells in which the
chromosomes are paired up and have
a partner in size and shape.
–
A cell containing TWO sets of
chromosomes (2n), one set inherited from
each parent is referred to as a DIPLOID
CELL.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Human female chromosomes shown by bright field G-banding
Where are the
homologous
chromosomes?
Homologous Chromosomes in Human Female
Review: DIPLOID Cells
• Each pair of homologous chromosomes
includes one chromosome from each parent
• The 46 chromosomes in a human somatic cell
are two sets of 23: one from the mother and
one from the father
• A diploid cell (2n) has two sets of
chromosomes
• For humans, the diploid number is 46 (2n = 46)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Key
2n = 6
Maternal set of
chromosomes (n = 3)
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosome
Two nonsister
chromatids in
a homologous pair
Centromere
Pair of homologous
chromosomes
(one from each set)
Sex Chromosomes v. Autosomes
• The sex chromosomes are called X and Y
• Human females have a homologous pair of
X chromosomes (XX)
• Human males have one X and one Y
chromosome (XY)
• The 22 pairs of chromosomes that do not
determine sex are called autosomes or
autosomal chromosomes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Haploid
• A gamete (sperm or egg) contains a single set of
chromosomes, and is haploid (n)
•
For humans, the haploid number is 23 (n = 23)
•
Each set of 23 consists of 22 autosomes and a single
sex chromosome
•
In an unfertilized egg (ovum), the sex chromosome
is X
•
In a sperm cell, the sex chromosome may be either X
or Y
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Review: HAPLOID Cells
• HAPLOID – means “one set”;
this describes the sex cells or
gametes (EGG AND SPERM)
– these contain only a
SINGLE set of chromosomes.
–
So, for a human, somatic
cells are diploid and have a
chromosome number of 46 (23
pairs).
–
But, sex cells are haploid,
and have a chromosome
number of JUST 23.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
So how do we transmit all of this information to
our offspring? A process called…
• Meiosis is the process by which the number
of chromosomes per sex cell is cut in half
through the separation of homologous
chromosomes in a diploid cell
– in other words, meiosis is the division of
chromosomes into gametes  eggs (in
females) and sperm (in males)
Key
Haploid gametes (n = 23)
Haploid (n)
Egg (n)
Diploid (2n)
Sperm (n)
MEIOSIS
Ovary
FERTILIZATION
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)