Unit 8: The Cell Cycle. Why do Cells Divide? . To make copies of cells for growth (you started out as 1 cell, today you have around 10 trillion) . Why do cells divide?. To make copies of cells for repair (cells wear out and need to be replaced). Why do cells divide? .
To make copies of cells for growth (you started out as 1 cell, today you have around 10 trillion)
To make copies of cells for repair (cells wear out and need to be replaced)
To carry on the species – during ASEXUAL reproduction organisms undergo MITOSIS to make exact copies of themselves to create offspring.
During SEXUAL reproduction two gametes (egg and sperm) get together to create offspring. Gametes are made through a process called MEIOSIS.
In eukaryotic cells, the genetic information that is passed from one generation of cells to the next is carried by chromosomes.
Chromosomes are made up of DNA.
Chromosomes are not visible in most cells, except for during cell division. At the beginning of cell division, chromosomes condense and become visible.
Before cell division, each chromosome is replicated (remember DNA replication).
Because of this each chromosome is made of 2 identical “sister” chromatids.
When the cell divides the “sister” chromatids separate from each other. One chromatid goes to each of the two new cells.
Each pair of chromatids is attached at an area called the centromere (usually in the middle of the chromatids)
Humans have 46 chromosomes. There are 23 pairs. 23 chromosomes come from their father and 23 from their mother.
2 of the chromosomes are sex chromosomes (represented by an X and Y).
The cell cycle is the series of events that cells go through as they grow and divide.
During the cell cycle a cell:
prepares for division
and divides to form two new “daughter” cells.
Interphase - the non-dividing part of the cell cycle, and it consists of G1 phase, S phase, and G2 phase.
Cyclins – a group of proteins that regulates when a cell should divide.
Cancer cells do not respond to the signals that regulate the growth of cells.
Mitosis – cell division in eukaryotic cells that creates two identical daughter cells that are diploid (2N).
Diploid – the cells have two sets of chromosomes (example: humans have 46 chromosomes, 23 pairs)
The cell grows and replicates its DNA, organelles, and centrioles
Step 1: Prophase
The chromatin condenses into chromosomes.
The centrioles separate, and a spindle begins to form.
The nuclear envelope breaks down.
Step 2: Metaphase
The chromosomes line up across the center of each cell.
Each chromosome is connected to a spindle fiber at it’s kinetochore (part of the centromere)
Step 3: Anaphase
The sister chromatids separate into individual chromosomes and are moved apart.
Step 4: Telophase
The chromosomes gather at opposite ends of the cell and lose their shapes.
Two new nuclear envelopes will form.
In animal cells a cleavage furrow pinches the cytoplasm into 2 new cells.
In plants a cell plate forms to divide the cells.
2 identical diploid daughter cells are formed.
P – Prophase – Prepare
M – Metaphase – Meet in middle
A – Anaphase – Apart
T – Telophase – Two new nuclei
ex: hair, blood, skin
46 chromosomes = ____ = _______
ex: sperm (males), eggs (females)
23 chromosomes = __ = _______
The Cell Cycle is the series of events that cells go through as they _________ and __________.
There are 4 phases:
Meiosis – cell division that creates 4 different daughter cells that are gametes (sex cells).
Each gamete created is haploid (1N), it only contains one set of chromosomes.
In humans, the gametes (sperm and egg) each contain 23 chromosomes.
When fertilization occurs, then the resulting cell has 46 chromosomes (23 from the sperm and 23 from the egg).
Each chromosome pairs with its matching homologous chromosome to form a tetrad.
Crossing over occurs between the chromatids.
The homologous chromosomes line up in the middle. Spindle fibers attach to each of the homologous chromosomes.
The spindle fibers pull the homologous chromosomes towards the opposite ends of the cell.
Telophase 1 and Cytokinesis
Nuclear membranes form.
The cell separates into two new cells.
The chromosomes condense and become visible.
NO crossing over occurs.
The chromosomes line up in the middle.
The sister chromatids separate and move to opposite ends of the cell.
Telophase 2 and Cytokinesis
Meiosis 2 results in four daughter cells that are haploid (1N)
Tetrad – four sister chromatids (2 homologous chromosomes) that match up with each other during prophase 1.
Crossing over – When homologous chromosomes switch part of their chromatids. Results in genetic variation.
Homologous chromosomes: chromosomes with the same genes as each other, in the same sequence, but do not necessarily have the same allele of those genes.
Homologous chromosomes pair up with one another, forming a tetrad (4 chromatids) during prophase I.
This occurs at the stage when chromatids of homologous chromosomes pair up during synapsis, forming a tetrad. The chromatids form X-like structures (chiasma or chiasmata). They break into segments, which are then exchanged with one another.
Crossing over is important because it results in new combinations of genes that are different from either parent, contributing to genetic diversity (why you look different than your parents & siblings)
When a zygote is formed from the fertilization of a gamete with an extra chromosome, three chromosomes of one type are present instead of two (Ex: Down syndrome (trisomy 21, 47 chromosomes) Klinefelter’s Syndrome in males ( XXY, 47 chromosomes)
When a zygote is formed from the fertilization of a gamete with one less chromosome, 1 chromosome is present instead of two (monosomy) ( ex: Turner’s Syndrome in females (XO, 45 chromosomes)