L10 cell divison

what are the 3 fundamental process genetic information is maintained from cell to cell

replication transcription translation

DNA Replication - overview

Replication begins at a particular point on the chromosome called the origin of replication

The DNA double helix unwinds, breaking the hydrogen bonds that join the base pairs, and forming two separate strands.

New strands of DNA are built using the old strands as templates. With the four different nucleotides attach themselves to the bases on the old strands by complementary base pairing

This synthesis of a new strand is done by an enzyme called DNA polymerase

You now have 2 double stranded molecules identical to the old molecule.

why is DNA replication referred to as semi conservative?

This is because when the double helix replicates, each of the two daughter molecules will have one old strand, derived from the parent molecule and one newly made strand

why is DNA replication required for cell division

it allows for a 'copy' of the genome to be made and for two daughter cells to be made.

what does the maintenance and reproduction of biological organisms depend on?

the ability of a single cell to divide

Cell division in prokaryotes(bacteria)

binary fission

Cell Division in multicellular eukaryotes

mitosis

meiosis

what happens in all types of cell division

each new cell produced must receive a copy of all the genetic information (DNA) in the parent cell.

mitosis

- occurs in non-reproductive cells (somatic), for the purpose of growth and repair

meiosis

occurs in reproductive cells(germ line cells), for the purpose of production of gametes, i.e. egg or sperm

where does cell division in plants occur

meristems in tips of roots and shoots

4 stages of the cell cycle

G1 (gap1) S (synthesis) G2 (gap2 ) M (mitosis)

G1: gap 1

•Cells increase in size and synthesize proteins, including various enzymes that are required for DNA replication in S phase •A control mechanism (G1 checkpoint) ensures that the DNA is ready for replication in the S stage. During G1 some cells may enter a phase known as G0 and are referred to as quiescent i.e. they where they no longer divide. This may be a temporary stage (liver cells) or permanent as in nerve cell.

S: synthesis

•In this stage DNA replication takes place, i.e. all the DNA in the nucleus is copied. •This is necessary to ensure that each new daughter cell gets an exact copy of the DNA present in the original parental cell. •In DNA replication the double-stranded DNA unwinds and each strand is copied separately to give 2 identical copies of the original parental DNA.

G2: gap 2

•G2 is another gap stage where the cell continues to grow producing new proteins necessary for mitosis. •Another control mechanism (G2 checkpoint) checks that the process of DNA replication is complete and the cell is ready for mitosis. The above 3 phases are collectively referred to as Interphase

M-stage

1. Prophase, 2. Metaphase 3. Anaphase 4. Telophase. •Mitosis is strictly nuclear division, and is followed by cytoplasmic division, or cytokinesis, to complete cell division. •Mitosis results in two "daughter cells", which are genetically identical to each other, and is used for growth and repair. •It occurs in the somatic cells (i.e. non-reproductive cells) of an organism

interphase

•Between mitotic divisions a cell is in interphase with the genetic material in the form of chromatin. •Interphase is comprised of G1, S and G2 phases of the cell cycle.

early prophase

Nucleus enlarges and chromatin condenses to chromosomes, which are visible by light microscopy.

•Each chromosome consists of two sister chromatids attached by a centromere

late prophase

Chromosomes shorten and thicken. •The nuclear membrane breaks down. •Centrioles move to opposite ends of the cell. These will form the spindles required during metaphase.

metaphase

•A spindle of microtubules forms from the poles of the cell.

•As a result, all chromosomes become aligned halfway between the poles, at the cell equator.(metaphase plate)

•The chromatids of each chromosome become oriented and are attracted towards opposite poles of the cell.

anaphase

•The spindle contracts •Centromeres split apart pulling the chromatids to opposite ends of the cell •At this point each chromatid referred to as a daughter chromosome

telophase

•The chromosomes de-condense into chromatin. •The nuclear envelope reforms, giving a cell with two nuclei.

cytokinesis

•The cell membrane is constricted across the equator, thus splitting the cytoplasm in two and two new cells are formed which then enter the G1 phase of interphase.( or G0 if they have finished dividing

function of mitosis in multi-cellular organism

growth and repair

what does meiosis result in

the formation of reproductive cells called gamete s ...egg/sperm

diploid

there are two chromosomes of each type present in each cell.

homologous chromosomes

one inherited from the father and one from the mother

how many pairs of homologous chromosomes do humans have

23 pairs with a total of 46

1-22 pairs of chromosomes

autosomes

23rd pair of chromosomes

sex chromosomes

female sex chromosome

two X chromosomes(XX

male sex chromosome

there is one X chromosome and one Y chromosome (XY

what does meiosis reduce

the diploid number of chromosomes in half, to a haploid number.

haploid

there is only one chromosome of each type present in each cell.

division of a reproductive germ cell

each gamete (sperm/egg) ends up with one member of each of the pairs of homologous chromosomes in the germ cell.Thus in human gametes (sperm/egg) there are only 23 chromosomesAt fertilization the diploid number of chromosomes (46) is restored in the fertilized egg.

meiosis sets

Meiosis I (Prophase, Metaphase, Anaphase and Telophase I)

Meiosis II (Prophase, Metaphase, Anaphase and Telophase II) ....... It results in the formation of 4 new cells, each with a haploid number of chromosomes

Prophase I

Chromosomes condense and become visible as in mitosis Homologous chromosomes pair up together.Each chromosome is seen to consist of two sister chromatids. Chromatids can break at different points and exchange genetic material with non-sister chromatids -referred to as Crossing-over.The points on the chromosome at which the crossing over takes place are called chiasmata

metaphase I

•Nuclear membrane disappears. •Spindle forms •The pairs of homologous chromosomes line up randomly along the cell equator

Anaphase I

•The homologous chromosomes migrate and separate to opposite poles.

•Each chromosome is still composed of sister chromatids and 'crossed over" parts.

•In humans this means that 23 chromosomes go to each pole and that the homologous pairs are forever separated.

•This phase is often referred to as 'reduction division' since chromosome number is now halved.(haploid)

telophase I

•Chromosomes decondense, nuclei reorganize, nuclear membrane forms, •Cytoplasm divides (cytokinesis), which results in two new cells.(haploid) •Note the sister chromatids are still together

prohase II

Prophase II is the start of Meiosis II. Meiosis II is simpler thanMeiosis I in that it is essentially the same as mitosis.Chromatin condenses to chromosomes. Each chromosome consists of two sister chromatids attached by a centromere.The nuclear membrane breaks down.Centrioles move to opposite ends of the cell. Recall there are now two cells undergoing this process.

metaphase II

•A spindle of microtubules forms from the poles of the cell. •As a result, all chromosomes become aligned halfway between the poles, at the cell equator. •The chromatids of each chromosome become oriented and are attracted towards opposite poles of the cell.

anaphase II

•The spindle contracts, centromeres split apart pulling the chromatids to the opposite poles. •Each sister chromatid is brought to each end of the cell.(now referred to as a chromosome

Telophase II & Cytokinesis

•Chromosomes decondense and nuclear membranes forms around each pole area followed by cytokinesis. •Each cell has now half the number of chromosomes.

What two events in meiosis contribute to genetic variation ?

Crossing over

Independent assortment of chromosomes

crossing over

In prophase 1`of meiosis pairs of homologous chromosomes (one maternal one paternal) align closely to each other.Portions of non-sister chromatids get swapped at different points .Thus genes from paternal and maternal sources get combined on the one chromosome, increasing genetic variation.

Independent assortment of chromosomes

At Metaphase 1 of meiosis the homologous pairs of chromosomes, each consisting of one maternal and one paternal chromosome, are situated at the equator (metaphase plate).

orientations of the homologous pairs relative to the poles of the cell

they are random

possibilities of orientations of the homologous pairs

If there are 2 homologous pairs of chromosomes, the number of possible combinations is 4 (22). For humans with 23 pairs of chromosomes the number of possibilities thus is 223 (over 8 million!)

what does a DNA molecule consist of ?

consists of two polynucleotide strands wound around each other

Packing of genetic material with the eukaryotic cell

DNA packing is the process of tightly packing DNA molecule to fit into the nucleus of the cell. is very important as DNA is the blue print for the cell and requires all instructions required to function

3 fundamental processes Genetic information is maintained from cell to cell

replication

transcription

translation

evolutionary precursor to mitosis.

binary fission

how do prokaryotes reproduce

Prokaryotes reproduce asexually by cell division process called binary fission.

two types 'Cell division in multicellular organisms can be divided into

1. Mitosis- occurs in non-reproductive cells (somatic), for the purpose of growth and repair. 2. Meiosis- occurs in reproductive cells(germ line cells), for the purpose of production of gametes, i.e. egg or sperm.

cell division in plants

cell division is confined to specific areas called meristems found at the tips of roots and shoots

4 stages of the cell cycle

G1 (gap1)

S (synthesis)

G2 (gap2 )

M (mitosis)