Chapter 10 - Cells divide for growth, repair, replacement and reproduction

Average life span of human cells

The events that occur in the cell cycle have been divided into a number of phases:

The stages of mitosis in a cell with two chromosomes

Interphase

Prophase

Two pairs of centrioles become visible early in prophase. They move to opposite ends of the cell and microtubules begin to radiate from them. At the same time, the nucleolus disappears and the nuclear membrane begins to break down. The chromatin threads of DNA become tightly coiled and can be seen as chromosomes. Coiling the long, delicate DNA molecules makes it easier to distribute the DNA to the daughter cells. Each chromosome consists of two chromatids, which are joined at a point called the centromere. The two chromatids are identical, tightly coiled DNA molecules produced from DNA replication during interphase.

Metaphase

During metaphase, the chromatid pairs line up on the equator of the spindle. The centromere of each pair is attached to a spindle fibre.

Anaphase

In anaphase, each pair of chromatids separates at the centromere. As the chromatids have become independent of each other, they are now each called chromosomes. The new chromosomes are then pulled away from one another towards opposite poles of the cell. The centromeres are still attached to the spindle fibres, and it seems that the spindle fibres pull the chromosomes apart in some way.

Telophase

During telophase, the two sets of chromosomes form tight groups at each pole of the cell. A nuclear membrane forms around each group, and a nucleolus appears in each new nucleus. The spindle fibres disappear, and the chromosomes gradually uncoil to become chromatin threads once more.

Cytokinesis

Division of the cytoplasm is called cytokinesis. A furrow develops in the cytoplasm between the two nuclei. The furrow gradually deepens until it cuts the cytoplasm into two parts, each with its own nucleus. (Note: Although the term ‘mitosis’ is commonly used to refer to cell division, it technically refers just to the division of the nucleus.)

Summary of cell division

differentiation.

cells undergo division by mitosis, different genes become activated. This makes the cells differentiate into specialised cells that can perform particular functions

Stem cells

The cells that can undergo differentiation are called stem cells.

Totipotent stem cells

have the ability to form the embryo and the membranes that will surround, support and nourish it. An example is the early embryo before the formation of the inner cell mass.

Pluripotent stem cells

are capable of giving rise to most, but not all, tissues of an organism. An example is the inner cell mass.

Multipotent stem cells

are able to give rise to cells that have a specific function. An example is blood stem cells.

First division:

The homologous pairs separate and two daughter cells form with 23 chromosomes, each with two chromatids.

Second division:

The chromatids separate, resulting in four daughter cells with 23 chromosomes, each with one chromatid.

Summary of meiosis: one diploid parent cell produces four haploid daughter cells

Second meiotic division

Comparison of mitosis and meiosis

Crossing over

An important feature of meiosis occurs during the prophase of the first meiotic division. When the homologous chromosomes are paired, the chromatids may cross, break and exchange segments. This is called crossing over and the point where two chromatids cross is called a chiasma. Crossing over can result in a new combination of alleles along the chromosome. This is called recombination. Therefore, crossing over creates new combinations of genes so that the chromosomes passed on to the offspring are not exactly the same as those inherited from the parents.

Non-disjunction

During the first division of meiosis, the homologous chromosomes pair and then separate. Sometimes one or more of the chromosome pairs may fail to separate when the cell divides. In the second meiotic division, one or more of the chromatids may fail to separate. These situations are called non-disjunction, and they result in one of the daughter cells receiving an extra chromosome and the other daughter cell lacking that chromosome

Monosomy

Monosomy is where an individual is missing a chromosome – they have only one copy instead of the normal two. Like trisomy, monosomy usually results in severe malformations and often miscarriage.

Haploid number

The number of chromosomes in a cell with only one chromosome from each homologous pair; half the diploid number

Homologous chromosomes

The pairs of chromosomes containing genes that control the same characteristics

Independent assortment

The random combination of alleles due to allele pairs separating independently of one another

THE ROLE OF DNA IN CELL PROTEIN SYNTHESIS

 The genetic code in DNA provides the instructions for making cell proteins.

 As well as instructions, the cell needs reactants (amino acids) and energy (from respiration).

 Proteins are formed on the ribosomes in the cytoplasm.

 DNA does not leave the nucleus of the cell.

 The cell has a special way of transferring the genetic code to the ribosome.

Creating a useful copy of dna - mrna

Translation
building the protein

 LOCATION: Ribosomes

 KEY PLAYER: tRNA – brings the corresponding amino acid to the ribosome.

Ploidy

´Ploidy is the number of pairs of chromosomes in a cell.

Genetic loci

A genetic location is called a locus.

A locus is the position of a gene, a cluster of genes or even a single nucleotide on a chromosome.

Since there are two of each chromosome in a somatic cell, there are two copies of each gene.

CELL DIVISION BY MITOSIS

´In mitosis cells are produced for growth, maintenance and repair of the body.

´The more wear & tear on a cell, the shorter its life span.

´Intestinal cells last 1.3 days

´Brain nerve cells last a lifetime.

The cell cycle

G0 phase occurs

G0 phase occurs when the cell has

exited the cell cycle and is not preparing

to undergo division.

Cells in G0 phase can sometimes

re-enter the cell cycle if they receive the

correct stimuli.

Cells that have terminally

differentiated (i.e. reached their

most specialised, final form such

as a nerve cell) are usually in

G0 phase.

Interphase

PROPHASE

METAPHASE

ANAPHASE

TELOPHASE

CYTOKINESIS

Gene expression

 Protein synthesis is also known as gene expression.

 Only certain genes are expressed in a cell at any given time.

 Gene expression can be switched ‘on’ or ‘off’, depending on the cell’s requirements.

Epigenetics and gene expression

 Epigenetics refers to differences in traits caused by environmental factors that can be inherited.

 The differences are not due to changes in the DNA sequence, but due to modifications, such as methylation of the DNA or histone modification.

 This changes the gene expression without changing the DNA i.e. genes are more likely to be turned on or off.

Dna methylation

 Methyl groups are clusters of hydrocarbons which attach to DNA strands

 When a methyl group is attached to a particular gene, the gene cannot be expressed. i.e. the gene is “switched off”.

Histone modification

 Refers to how tightly the chromatin is wound around the histone protein.

 Tightly wound chromatin stops access for RNA polymerase, stopping gene expression.

What are stem cells?

three main types of stem cells

Multipotent

Pluripotent

Totipotent

Mutipotent

§Multipotent – differentiate in to more than one type of body cell

Pluripotent

 able to differentiate, or mature, into the three primary groups of cells that form a human being: (ectoderm — giving rise to the skin and nervous system, endoderm — forming the gastrointestinal and respiratory tracts, endocrine glands, liver, and pancreas and mesoderm — forming bone, cartilage, most of the circulatory system, muscles, connective tissue, and more)

Totipotent

§Totipotent - give rise to any of the 220 cell types found in an embryo as well as extra-embryonic cells (placenta).

Source of stem cells:
embryonic stem cells

 Embryonic stem cells are pluripotent. They can differentiate into all types of body cell.

 Taken from the inner cell mass of a 5-7 day old embryo (formed by in-vitro fertilisation).

 Embryo is destroyed in the process.

 One embryo can give rise to a line of pluripotent stem cells.

 Strict controls for their use.

SourceS of adult stem cells
1. Umbilical blood & Placental cells

 Adult stem cells refers to a human of any age after cell differentiation.

 Multipotent adult stem cells can be obtained from umbilical cord blood and placental stem cells.

 These tissues can be donated to the national cord blood blank or stored in case the child needs it in the future.

Sources of adult stem cells
2. body organs

 These multipotent cells are found in most organs of the body.

 They usually only form the tissues of the organ from which they are found.

 Harvesting and culturing a patient’s own stem cells prevents tissue rejection.

TERMS ASSOCIATED WITH  GAMETE PRODUCTION

• Gametogensis

• Spermatogenisis

• Oogenesis

Gametogenesis

 Gametogenesis – process of gamete development.

Spermatogenesis

 Spermatogenesis – process of spermatozoa development in male testis.

Oogenesis -

 process of ova development in female ovary.

HOMOLOGOUS CHROMOSOMES

 At fertilisation, 23 chromosomes from a (haploid) ovum unite with 23 chromosomes from a (haploid) spermatozoon to form a (diploid) human cell with 46 chromosomes.

 Thus diploid cells have 23 chromosome pairs.

 Each member of a pair carries genetic information for the same features.

 They are called homologous chromosomes.

 Humans have 23 pairs of homologous chromosomes.

MEIOSIS

 Specialised form of cell division used only in gamete production (ova & sperm).

 Results in halving of cell chromosome number in humans from 46 to 23.

 Diploid cell chromosome number = 46.

 Haploid cell chromosome number = 23.

 At fertilisation, ova & sperm nuclei fuse to restore diploid chromosome number.

 New human develops with 46 chromosomes.

FIRST MEIOTIC DIVISION – REDUCTION IN CHROMOSOME NUMBER FROM DIPLOID TO HAPLOID

Secound MEIOTIC DIVISION – REDUCTION IN CHROMOSOME NUMBER FROM DIPLOID TO HAPLOID

Third MEIOTIC DIVISION – REDUCTION IN CHROMOSOME NUMBER FROM DIPLOID TO HAPLOID

Fourth MEIOTIC DIVISION – REDUCTION IN CHROMOSOME NUMBER FROM DIPLOID TO HAPLOID

Fifth MEIOTIC DIVISION – REDUCTION IN CHROMOSOME NUMBER FROM DIPLOID TO HAPLOID

SECOND MEIOTIC DIVISION – SEPARATION OF CHROMATIDS. 1

SECOND MEIOTIC DIVISION – SEPARATION OF CHROMATIDS. 2

SECOND MEIOTIC DIVISION – SEPARATION OF CHROMATIDS. 3

SECOND MEIOTIC DIVISION – SEPARATION OF CHROMATIDS. 4