Download CHAPTER 1 Genetics: An Introduction

The Continuity of Life
How Cells Reproduce
• Cell division is at the heart of the
reproduction of cells and organisms
• Organisms can reproduce sexually or
asexually.
• Some organisms make exact copies of
themselves, asexual reproduction.
• Other organisms make similar copies of
themselves in a more complex process,
sexual reproduction
• Cells arise only from preexisting cells
• Cellular reproduction is called cell division
– Cell division allows an embryo to develop into
an adult
– It also ensures the continuity of life from one
generation to the next
What Occurs During the Prokaryotic
Cell Cycle?
Prokaryotic cells divide asexually
– These cells possess a single
chromosome, containing genes
– The chromosome is replicated
– The cell then divides into two cells, a
process called binary fission
Cell division
cell
division
cell growth and
DNA replication
What Occurs During the Eukaryotic Cell
Cycle?
• The cell cycle consists of two major phases:
–Interphase, where chromosomes duplicate and
cell parts are made.
–The mitotic phase, when cell division occurs
–Interphase consists of three main stages:
G1 or Gap 1
S or Synthesis
G2 or Gap 2
The chromosomes of eukaryotes
duplicate with each cell division
• A eukaryotic cell has many more genes than
a prokaryotic cell
– The genes are grouped into
multiple chromosomes,
found in the nucleus
– The chromosomes of this
plant cell are stained
dark purple
• Chromosomes contain a very long DNA
molecule with thousands of genes
Sister chromatids
– Individual chromosomes are
only visible during cell
division
– They are packaged as
chromatin.
– Before a cell starts dividing,
during S, the chromosomes
are duplicated. This process
produces sister chromatids
Centromere
• There Are Two Types of division in Eukaryotic
Cells:
Mitotic Cell Division and Meiotic Cell Division
• Mitotic cell division is the type of cell division
when a mother cell produces two genetically
identical daughter cells. For example: If the
mother cell has 46 chromosomes the daughter
cells will also have 46 chromosomes.
• Cell division has two steps: Mitosis or division of
the nucleus and cytokinesis or division of the
cytoplasm.
• Mitosis has four phases: Prophase, Metaphase
• Anaphase and Telophase. Cytokinesis begins
at the end of anaphase
• When the cell
divides, the sister
chromatids
separate
– Two daughter
cells are
produced
– Each has a
complete and
identical set of
chromosomes.
Each
chromosome has
only one
chromatid.
Chromosome
duplication
Sister
chromatids
Centromere
Chromosome
distribution
to
daughter
cells
• The process of mitotic cell division is used
for growth, healing, repair and
regeneration.
Meiosis
• Meiosis is the process of cell division that we
use for the formation of sex or reproductive cells.
• In humans, Meiosis takes place in the
reproductive organs: the testis in males and the
ovaries in females.
• In plants, Meiosis takes places in the anthers
(male structures) and in the ovary (female
structure) of the flower.
• When a mother cell divides, it produces four
genetically different daughter cells with half the
number of chromosomes than the mother cell.
Chromosomes are matched in
homologous pairs
• Somatic cells of
each species
contain a specific
number of
chromosomes
– Human cells have
46, making up 23
pairs of homologous
chromosomes
Chromosomes
Centromere
Sister chromatids
Gametes have a single set of
chromosomes
• Cells with two sets of chromosomes are said
to be diploid
• Gametes are haploid, with only one set of
chromosomes
• The human
life cycle
Haploid gametes (n = 23)
Egg cell
Sperm cell
MEIOSIS
FERTILIZATION
Diploid
zygote
(2n = 46)
Multicellular
diploid adults
(2n = 46)
Mitosis and
development
• At fertilization, a sperm fuses with an egg,
forming a diploid zygote
– Repeated mitotic divisions lead to the
development of a mature adult
– The adult makes haploid gametes by meiosis
– All of these processes make up the sexual life
cycle of organisms
mitotic cell division,
differentiation, and growth
adults
baby
mitotic cell division,
differentiation,
and growth
meiotic cell
division in
ovaries
embryo
egg
mitotic
cell division,
differentiation,
and growth
fertilized
egg
sperm
fertilization
meiotic cell
division in
testes
Independent orientation of
chromosomes in meiosis and
random fertilization lead to varied
offspring
• Each chromosome of a homologous pair
comes from a different parent.
• Each chromosome thus differs at many
points from the other member of the pair
Homologous chromosomes carry
different versions of genes
• The differences between homologous
chromosomes are based on the fact that
they can carry different versions of a gene at
corresponding loci
Coat-color genes
Eye-color genes
Brown
Black
C
E
c
e
White
Pink
Tetrad in parent cell
(homologous pair of
duplicated chromosomes)
C
E
C
E
c
e
c
e
Chromosomes of
the four gametes