Cell Reproduction

Cell Reproduction

• The continuity of life is a result of cell division, which is one of the most distinctive characteristics of living organisms.

• In the cases of humans, we develop from a single cell that was originally the product of the fusion of two special cells, a sperm cell and an egg cell during sexual reproduction.

• Other organisms such as bacteria produce by asexual means, dividing rapidly in suitable conditions to produce millions of cells.

• Many fungi, liverworts and ferns also reproduce asexually, resulting in clones of genetically identical offspring.

Bacteria• Bacteria divide in a process known as binary

fission. • Each cell has a single circular chromosome which

attaches to the cell membrane at a specific site. • The DNA molecule replicates, and as the bacterial

cell grows, the two copies of DNA separate. • The cell cytoplasm then divides, and the cell

membrane and wall grow to separate the parts into two daughter cells.

• Unlike eukaryotes, the DNA does not condense during division.

The Cell Cycle

• The period between the formation of a new cell and when it divides to produce two daughter cells is called the cell cycle.

• In eukaryotes, actively growing and dividing somatic cells (non-sex cells) move through a series of phases in the cycle.

• Most of the time the cells are in interphase, the stage during which molecules are synthesized and DNA replicated.

• Interphase alternates with cell division or mitosis.

The Cell CycleInterphase• Divided into three stages

– Pre-DNA synthesis (G1)– DNA synthesis (S)– Post DNA synthesis (G2)

• Chromosomes are not visible during interphase

Cell Division• Divided into two stages:

– Mitosis (M) – division of nucleus containing chromosomes– Cytokinesis – division of the cytoplasm

• Mitosis occurs after G2• Cytokinesis occurs near the end of M.• In dividing cells, chromosomes are readily observed using a microscope. • Once cell division is complete, cells enter G1. • All cells produced during the mitotic cell cycle are genetically identical, cells

differentiate during G1 and G2.

Mitosis

• Each cell (not including sex cells) contains two of each chromosome, forming homologous (matching) pairs.

• Human cells somatic cells contain 46 chromosomes (23 matching pairs).

• Mitosis is the process by which chromosomes duplicate, sister chromatids separate and the nucleus of the cell divides. The genetic information in the two daughter nuclei is identical to that of the parent nucleus.

• NB – Interesting exception to remember are nematodes. Nematodes don’t undergo mitosis for growth and development. Each worm is a tiny, fully formed adult when hatched. All growth after this stage is due to an increase in size of individual cells.

Mitosis

• Stages of Mitosis:– Interphase– Prophase– Metaphase– Anaphase– Telephase

• Although mitosis is divided into different stages, it is important to realize that it is in fact a continuous process.

Stages of Mitosis

Interphase

• Chromosomes are not visible

• Cells are replicating

Prophase

• Chromosomes begin to shorten and thicken. They become visible under the microsope.

• Chromosomes are double stranded at this stage because DNA replicated during S phase, resulting in two identical copies of the chromosomes.

• These two copies called chromatids are held together by a constricted region called the centromere.

• Late in prophase the nuclear membrane disappears and a network of fibres known as the spindle appears. The spindle extends between the two poles of the cells.

Metaphase

• Chromosomes gather at the equator (central region) of the spindle.

• The centromere of each chromosome attaches to the spindle fibres.

Anaphase

• Spindle fibres contract, pulling the centromere of each chromosome in two directions.

• Splitting of the centrome separates the chromatids into single strands.

• The separated chromatids are drawn to opposite poles of the cell.

• Now single, the separated chromatids are once again called chromosomes.

Telephase

• This is last stage of mitosis.• Nuclear membranes reform around each

group of chromatids.• Once mitosis is complete, the cytoplasm

divides by the process of cytokinesis. This separates the two daughter nuclei into separate cells.

• In animals the cell membrane pinches in, dividing the cell into two daughter cells.

• In plants, a new cell wall is laid down between the daughter cells. The components of the wall are initially deposited in the centre of the cell. Growth of the wall extends until the two daughter cells are completely separated.

Memory Aids for Mitosis

To remember the order in which steps occur:• Indian People Make Artistic Teepees• Interphase → Prophase → Metaphase → Anaphase → Telephase

To remember what occurs at each stage:• P – is for Pairs. We can see pairs of chromosomes under the

microscope• M – is for Middle. Sister chromatids gather in the middle of the

spindle• A – is Apart. Spindle contracts and chromatids are pulled apart• T – is for Tidy up. Cell is tidied up – new nuclear membranes are

formed

Mitosis

Meiosis• Meiosis is the process of cell division

that produces gametes (germ cells), the specialized cells that combine in sexual reproduction.

• Meiosis is a reduction division. • It involves one replication of DNA and

two nuclear divisions that result in halving the number of chromosomes (2n) in a parent diploid nucleus to four haploid (n) daughter nuclei in gametes.

Meiosis

Meiosis• During the first meiotic division, homologous

chromosomes align and crossing over occurs. • Crossing over involves the breaking and re-joining

of chromatids and therefore DNA molecules. • It takes place between two non-sister chromatids of

a chromosome pair. • The two chromosomes of a pair are held together at

the site of crossing over by a chiasma. A chiasma connects the chromatids until it is time for them to separate. Along the length of a long chromosome there may be several chiasmata.

• Crossing over produces chromosomes with new combinations of genetic information. This process is known as recombination.

Meiosis

• Homologous chromosomes line up together; chromatids break where they are twisted

• Chromatid ends join to 'wrong' pieces

• Homologous chromosomes move apart

• Separated chromosomes carry new gene combinations

Meiosis

Meiosis in Brief – First Meiotic Division

• Prophase I – Chromosomes shorten and thicken. – Homologous pairs of chromosomes pair with each other and crossing

over occurs. – Spindle is formed.

• Metaphase I– Homologous pairs align at equator of spindle. – They remain attached at crossing over points (this attachment is

random).• Anaphase I

– Homologous chromosomes separate. – As they pull apart points of crossing over are visible as chiasmata. – One member of each homologous pair migrates to each pole of the cell.

• Telephase I– nuclear membrane begins to reform. – Each new nuclei has one member of each homologous pair of

chromosomes (n) but each chromosome still consists of two chromatids. – Cytokinesis occurs.

• Interkinesis– Brief phase between divisions. – No replication occurs. – Interkinesis absent in some organisms.

Meiosis in Brief – Second Meiotic Division

• Prophase II– Nuclear membrane breaks down, – Centrioles divide again and spindle reforms.

• Metaphase II– Paired chromatids align at equator of spindle

• Anaphase II– Centromeres divide and chromatids separate and move to opposite poles

of each cell.• Telephase II

– Nuclear membrane reforms. – Spindle breaks down and cytokinesis occurs.

• End product is four gametes each of which is different. • Each gamete contains one member of each homologous pair• Usually in females three of the new nuclei are broken down and only

one survives in the newly formed ova.

Summary of inputs and outputs for meiosis

Meiosis

Important side note to meiosis

• Mitochondria and chloroplasts have a circular molecule of DNA within them.

• As mitochondria and chloroplasts are part of the cytoplasm, they are passed from generation to generation via one parent, usually the female.

• The inheritance of mtDNA or cpDNA from one parent is called uniparental inheritance.

• This is important when we consider some genetic disorders.– Leber’s Hereditary Optic Neuropathy is due to a mutation in

mtDNA and results in mitochondria producing less ATP. As optic cells need high levels of ATP they are the first to die.

• Very important to realize that the inheritance of faulty mitochondria is random due to the nature of cytokinesis.

Problems with Meiosis

• Meiosis is usually an exact process, but sometimes errors occur.

• The resulting chromosomal abnormalities (extra or missing chromosomes) can have severe effects on off-spring.

• Non-separation of chromosomes, and fusion of non-homologous chromosomes can both occur during meiosis and lead to different problems.

• The inheritance of too many or too few chromosomes is referred to as aneuploidy, and can occur in autosomes or in sex chromosomes.

• About 15% of pregnancies end with a spontaneous abortion. Approximately half of these occur because the zygote has an abnormal number of chromosomes.

Problems with Meiosis:Down Syndrome

• Down syndrome is the result of having three copies of chromosome 21 and is often referred to as trisomy-21.

• It can occur by chance (non-familial Down Syndrome) or there may be a history of the syndrome in the family.

• The extra copy of chromosome 21 usually results from an error in meiosis of one of the parents.

• Chromosome 21 undergoes nondisjunction so that cell division produces gametes with either an extra or missing chromosome 21.

• If a gamete with (N + 1) chromosomes unites with a normal gamete (N) the zygote will be trisomic (2N + 1).

• People with mild Down syndrome are often mosaics, some of their cells carry 3 copies of chromosome 21 and others are normal. The severity of the syndrome will then depend on which cells are affected.

Unusual example of Familial Down Syndrome

• A child with familial Down syndrome had the same number of chromosomes (46) as individuals of normal chromosome complement, rather than the 47 chromosomes usually observed in Down syndrome

• Parents of these children were of normal phenotype, but had chromosome numbers of 46 and 45.

• Closer examination of the chromosomes of the parent with 45 showed that his cells had only one copy of chromosome 15 and one copy of chromosome 21, rather than the expected two copies of each.

• In addition, this parent had a ‘hybrid’ chromosome (termed 15/21) consisting of a chromosome 15 and a chromosome 21 joined together (called a translocation).

• The child with Down syndrome was found to have one copy of chromosome 15, two copies of chromosome 21 and one copy of chromosome 15/21.

• Therefore the child had effectively two copies of chromosome 15 and three copies of chromosome 21.

• The outcome is the same as when trisomy-21 occurs due to nondisjunction, the only difference being that the chromosomes are ‘packaged’ in a different way.

Unusual example ofFamilial Down Syndrome

Polyploidy• Polyploidy refers to having more than two sets of chromosomes (eg

3N, 4N, 6N) in a genome.

• Polyploidy may arise due to errors in meiosis – e.g. a gamete may be diploid instead of haploid. If a diploid sperm fertilizes a haploid egg the resulting zygote has one extra set of chromosomes and is therefore triploid (3N).

• Polyploid zygotes don’t survive in humans, however, polyploidy can also arise during mitosis and produce groups of somatic polyploidy cells – this does not necessarily affect health.

• Polyploidy is much more common in plants because many plants can survive by asexual reproduction. Polyploid plants may be sterile because of problems with chromosome pairing during meiosis, but continue to survive by vegetative reproduction.

• Examples of polyploidy in plants: some bananas (3N), cultivated cotton and potatoes (4N), strawberries (8N)

• Examples of polyploidy in animals: some insects, earthworms and tree frogs

How polyploidy occurs

(a) A tetraploid organism (4N) can be the result of the chance doubling of chromosomes.

(b) A tetraploid produces 2N gametes. If the tetraploid mates with a normal diploid (1N gametes), then the offspring will be a sterile triploid (3N). However, two diploid gametes may occasionally fuse, producing an offspring with twice the number of chromosomes (a tetraploid, 4N).

Genes and Development• During embryonic development cells differentiate under the control of

genes into special cell types.• A master gene functions to regulate the activity of other genes.• These master genes are referred to as homeotic genes as their

products regulate the activity of other genes during development. • All homeotic genes contain a short sequence of DNA which codes for

a sequence of amino acids that bind to DNA. This binding allows the protein to regulate transcription of other genes.

• Some cells remain as undifferentiated stem cells, which can continue to replace themselves or differentiate into distinct cell types. These are referred to as stem cells.

• There are two types of stem cells: embryonic stem cells (found in embryos) and adult stem cells (found in mature tissues such as bone marrow).

• Stem cells provide an ability for growth, repair, replacement and regeneration.

• Stem cells can be potentially used as therapeutic agents, but there are technical and ethical issues to be considered.

Comparison ofembryonic and adult stem cells

Cell death – Apoptosis• Programmed cell death, or apoptosis, is a normal part of the

life of cells.

• Cell death occurs when the cell membrane shrinks, DNA fragments and lysosomes empty their contents into the cell causing the cellular components to be broken down.

• The dead cell is then consumed by phagocytes.

• Cell death is important for:– Developmental changes in growing embryos

– Ridding tissues of old, infected or damaged cells

– Removing immune cells which attack “self”

– Removing cells which have sustained DNA damage so that they do not continue to reproduce and form cancers

• Too much apoptosis can also have serious consequences – apoptosis is a major factor in Alzheimer’s disease

Apoptosis