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Cell Growth & Division. Do cells grow? Yes but with limitations if a cell grows to be too large it requires more energy and resources to maintain metabolism and homeostasis
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Do cells grow? • Yes but with limitations • if a cell grows to be too large it requires more energy and resources to maintain metabolism and homeostasis • As a cell becomes too large DNA (the brain of the cell) becomes overloaded much like a computer of limited memory trying to run a very large program.
As multicellular organisms grow, their cells multiply rather than growing indefinitely. • Before a cell becomes too large it undergoes cell division forming two new daughter cells. • This is a more efficient use of cell resources. • These new cells each get a full set of genetic information through the replication of DNA
Types of Cell Division • Prokaryotes divide by a process called Binary Fission. • In Eukaryotes there are 2 types of cell division • Mitosis: Results in 2 new identical cells each with a full set of chromosomes • Meiosis: Results in 2 cells with only half the original number of chromosomes (sex cells)
The DNA of the single chromosome is copied. • The cell grows to 2X its size • A cell membrane forms dividing the cell in half • the cell splits into 2 new cells each with its own chromosome
Prokaryotes such as Bacteria divide using binary fision • They have only singular circular DNA • Can divide and form new organisms every 20-30 minutes
The Role of Chromosomes • To understand cell division in eukaryotes it is important to first understand the role of Chromosomes. • During cell division DNA is tightly coiled with proteins into compact rod shaped structures called chromosomes within the nucleus. • The DNA is tightly coiled around proteins called histones which give the chromosome its shape.
During the time of cell life between divisions the DNA is less tightly coiled allowing its information to be “read” and processed by the cell. • In this loose form the DNA/protein complex is called chromatin. • As the DNA replicates it forms 2 identical halves of the chromosome called chromatids. • These halves are joined by a constricted area known as the centromere. • When the cell divides each new cell receives 1 chromatid or 1 copy of the DNA.
Each different species has a unique number of chromosomes. • man 46 • cats 32 • gorillas 48 • fruit fly 8
There are two types of human and animal chromosomes • sex chromosomes - these determine the sex of an organism but may also carry genes for other traits. In humans these are the X / Y chromosomes • autosomes – all of the other chromosomes that are not sex chromosomes. The majority of inherited trait are contained on these chromosomes.
Humans – • Autosomes 2 Sex chromosomes 46 total chromosomes
Every organism produced by sexual reproduction has 2 copies of each autosome. • One set is received from each parent. • These pairs of chromosomes are called Homologous chromosomes. • Each one of the chromosomes in a homologous pair is • the same size, • shape, • and carries genes for the same traits.
Humans – • homologous pairs of autosomes 1 pair of sex chromosomes 23 total pairs of chromosomes
Cells that contain 2 full sets of autosomes are said to be diploid. • All normal human cells are diploid meaning that they have a full 23 pairs of chromosomes. • The only human cells which are not diploid are the egg and sperm cells which have only half the number of chromosomes. Diploid cells are abbreviated as 2n
Reproductive or sex cells are called haploid cells because they have only 1 set of chromosomes and only 1 sex chromosome (X or Y). • Haploid cells do not have homologous pairs but merely 1 chromosome from each original pair Haploid cells are abbreviated as 1n
In sexual reproduction 2 haploid cells create a diploid cell. 1n + 1n = 2n • If sex cells were 2n the new cell formed from their union would have too many chromosomes.
Cell Cycle • Cell division may be more efficient but the phase of a cell’s life when it is dividing is relatively small compared to that of growth and development • This is the repeating set of events in the life of a cell is called the cell cycle. • Cell division is only a small part of that cycle. • The period of time between cell divisions is referred to as Interphase.
There are 4 phases of the cell cycle: • G1 phase – This is the time when new cells grow and mature • S phase – This is the period of time when DNA is replicated to prepare for cell division • G2 phase – The cell grows to 2x its size in preparation for cell division • M phase – consists of Mitosis,the process of nuclear division, and Cytokinesis, the process of cytoplasmic division. The first 3 phases of the cell cycle make up the period we refer to as interphase.
There are many times in which mature cells will leave the cell cycle during the G1 phase. • They enter what is known as the G0 phase. • Here cells merely exist, but many may return to the cell cycle in order to repair or replace damaged cells. • Mature human nerve cells will leave the cell cycle and stop dividing when they reach maturity. These cells generally do not return to the cycle.
