Cellular Reproduction and Chromosomes

Cell Cycle, Mitosis, Meiosis and Gene Expression

Fertilization

• When reproduction occurs there are two ways this can occur:– Asexual Reproduction

• This involves no fertilization, as offspring is produced by a single parent and inherit all the genes and traits from the single parent

– Sexual Reproduction• This involves fertilization of a female egg from

sperm cells of a male producing offspring of many traits.

• More on this later…

Cell Cycle

• Cells reproduce through a continuous sequence of growth and division known as the cell cycle

• Interphase – cells are making new molecules and DNA is copied in a process called DNA replication.– Interphase broken into 3 steps:

• G1 – Gap 1 involves metabolic activities• S – Synthesis involves DNA replication• G2 – Gap 2 is preparation for cell division• M – Mitosis and Cell Division

Cell Cycle

• Timing of each phase can vary among different types of cells.

– Organ cell vs. skin cell

• Parent cell divides to create two new daughter cells (somatic cells have 46 chromosomes)

• Purpose of Mitosis1. Growth

2. Regeneration of Damaged Tissue

3. Maintenance of the Body

Chromosomes

• Hold the genetic information needed to maintain the cell and make new copies of the cell

• Made up of two sister chromatids which are held together with a centromere.– Chromatids are exactly identical to one another

Phases of Mitosis• Several events must happen to ensure that as a

cell divides, the genetic material inside is properly shared between each new daughter cell.

• Root tip cells are often looked at as they undergo mitosis frequently

• Phase order: PMAT & C

1. Prophase2. Metaphase3. Anaphase4. Telophase

Cytokinesis

• p. 88, fig. 4

Prestep!!!• INTERPHASE!

• During this phase the cell grows, heals, and creates proteins in order to start the division process!

• The cell duplicates it’s genetic material (called chromatin) and creates two identical sister chromatids, joined by an object known as a centromere.

Mitosis

• Prophase • chromatin, which is DNA and

proteins, condenses and becomes visible

• Nuclear membrane and nucleolus disappear

• Centrioles made up of microtubules migrate to opposite poles of the cell

• Spindle fibres start to form between the two centrioles

Mitosis• Metaphase

• Spindle fibres attach to centromere

• Chromosomes line up on the cell’s equator (equatorial plate)

• Anaphase • Centromere splits and

chromatids are pulled to opposite poles of the cell

Mitosis• Telophase

• Chromatids reach the two opposite poles• At this time each chromatid is

considered a single non-replicated chromosome

• Chromosomes unwind and become less visible

• Cytokinesis• Actual cell division• Spindle fibres disappear, nucleolus

reappears, nuclear membrane and in plant cells a new cell wall is formed

Cell Clock

• Why is it that if cells can continuously divide, how come we can’t stay eternally young?

• Research has shown that there is a specific time frame, or biological clock, that regulates the amount of divisions a cell can make.

• Heart cells and the magic # of 50• Cell division is usually controlled by

specialization– i.e. Skin cells vs nerve cells

Cell Clock

• There are only two types of cells that are able to divide continuously– Spermatocytes (sperm producing cells)

• From puberty to old age, spermatocytes are produced• When it eventually becomes a sperm cell and specializes, there

are no more divisions that occur

– Cancer cells• Cancer cells grow so quickly that cell specialization does not

have time to occur. • i.e. Leukemia and white blood cells

Errors in Mitosis

• Mutations are permanent errors in the normal DNA molecule and can severely affect the mitotic process– Mutagens such as toxic compounds, radiation or

viruses can lead to mutations

• Mutations are passed on and only found in the daughter cells of the initially affected cell.

• FHIT gene affected by cigarette smoke will undergo mitosis more frequently then normal and this leads to a tumour

Errors in Mitosis

• Certain genes act like switches and can produce proteins that will turn certain processes like mitosis, on or off.

• A mutation could permanently affect one of these genes and leave a gene switched on permanently– Genes that are activated by a mutation are

called oncogenes and will often lead to tumours

Cancer

• Associated with many diseases but is based around the uncontrolled, unregulated growth of cells.

• After cells in your body specialize, they are only to divide to replace damaged cells.

• There is a balance between cell death and cell replication within the body to keep it healthy!!!

• Cancer disrupts this balance!!!

Cancer

• Cells usually don’t divide on their own• Cancer cells do!• Cancer cells can divide in culture about once every 24

hours. Not in living organisms though (thankfully!!!)• Cancer cells do not adhere to well to other cancer cells, nor

do they stick well to normal cells.• Metastasis!!!• Cancer cells lack the ability to mature and specialize:

Therefore another threat is that cancer cells cannot carry out some of the functions of normal cells.

Causes of Cancer

• Genetics plays a relatively small role in predicting cancer (breast cancer is one of the few cases where there is documented evidence)

• Lifestyle!!!

Cloning• Cloning is the process of forming IDENTICAL offspring

from a single cell or tissue. • For the most part, cloning does not result in the variation of

traits that would occur with the combination of male and female sex cells.

• Therefore what kind of reproduction is this?– Asexual

• Think about how cloning of cells is similar to cancer cells

What kind of clones can you have?• Plant Clones!!!!!• One of the biggest discoveries

in modern day genetics occurred in 1958 by Fredrick Stewart!

• He created a full carrot from a single carrot cell.

• This was the first instance of cloning and is now commonplace in orchids.

Genetic Engineering

• This discovery has lead to the process of Genetic Engineering!– What is genetic engineering?– The process of intentional production of new

genetic material by substituting or altering existing material.

However it does has it’s problems

– Carrots, ferns, tobacco petunias and lettuce clone well

– But grass and legume families don’t!!!!!– What has been found is that some clones turn

into roots, and others to leaves, which each uses different parts of DNA

– What has resulted in the theory that to clone plant cells well, the process of specialization/ differentiation must be delayed.

Animal Cloning

• During plant cloning experiments, Briggs and King were investigating nuclear transplants

• They extracted the nucleus from an unfertilized egg of a frog using a pipette, making the cell enucleated.

• Next the nucleus of a fertilized cell from the blastula stage and the nucleus was placed into the first cell.

• That cell began to act as a fertilized cell and the cell began to replicate.

• A nucleus that can bring a cell from egg to adult is referred to as totipotent.

Cloning from Adult Animals

• Originally, transferring nuclei from an adult cell (specialized) into an enucleated cell would not stimulate cell division– Why?

• Until recently the only way to clone was to split cells from embryos

• However, some cells would still specialize and full animals could not be cloned

Cloning

• Production of identical copies of molecules, genes, cells or even organisms.

• Ian Wilmut in 1997 = Dolly– Egg from one adult female sheep, removed

nucleus.– Took nucleus from the mammary gland cell of

another sheep and inserted it into the original.– Egg was then implanted into the uterus of a

surrogate mother sheep

Cloning

• Humans?– Any issues?

• Insulin = Gene cloning– Genes that produce insulin are introduced into the DNA of

bacterial cells. Insulin is then manufactured by the bacteria which we can harvest and use for diabetes patients.

• PCR – Polymerase Chain Reaction = Gene Cloning– A single gene, or less, can be copied– Used to amplify or create many copies of DNA

• Useful at crime scenes• Also useful to analyze ancient mummies DNA or in comparison of DNA of

extinct animals to those living today.

Meiosis, Chromosomes and Heredity

Meiosis and Chromosomes

• A zygote contains chromosomes from both parents but it does not contain double the number of chromosomes found in normal body cells. WHY?

• Meiosis only occurs in reproductive organs and produces cells known as gametes (eggs or sperm) which are haploid (n).– All other cells (somatic) are diploid (2n) and contain

two copies of each type of chromosome.– Page 161, table 5.1

Chromosomes

• The first part of meiosis reduces the chromosome number from diploid to haploid, known as reduction division.

• Each sperm or egg cell contains 22 autosomes and one sex chromosome (X or Y)– The autosomes control almost all of the functions of the

individual and the sex chromosomes determine the sex of the individual.

Reduction Division

Meiosis• Almost the same as mitosis however there are two

sequences of each of the phases.

• Interphase (see below)• Prophase I, Metaphase I, Anaphase I, and

Telophase I are all part of reduction division.• Prophase II, Metaphase II, Anaphase II and

Telophase II are identical to mitosis.

• Interphase – chromosomes replicate during interphase before cell division begins (sister chromatids joined together by a centromere).

Meiosis

• Prophase I – homologous chromosomes pair which make up four chromatids called a tetrad.– Homologous chromosomes are similar but not identical

(like a pair of shoes) and each one has come from each of your parents.

– During pairing a process known as crossing over can occur between non-sister chromatids.

– This allows for recombination of genes and contributes to genetic variation

– Page 163, fig. 5.14

Crossing Over

Meiosis• Metaphase I – spindle fibres attach to the centromere of

each chromosome

• Anaphase I - Homologous chromosomes are separated independently– The centromere does not split

• Telophase I• Needs to occur however can be a lengthy or short process

– Short cell division goes directly to meiosis II– Lengthy chromosomes uncoil and nuclear membrane is formed

(replication does not need to occur)• In females, meiosis II occurs after the egg is fertilized by a

sperm cell

Meiosis

• Each cell beginning meiosis II is haploid • Each cell at the end of meiosis II is also haploid

although they are called gametes or spores.– The exact process of gamete formation is talked about

later

• Meiosis II is exactly the same as Mitosis however there are only 23 chromosomes to split at the centromere in each cell instead of 46 as in the somatic cells

Gamete Formation

• Gametogenesis – process of creating sperm and eggs.– Spermatogenesis – male gamete formation

• Occurs in the testes• A diploid germ cell (spermatogonium) undergoes the

meiosis process to create 4 haploid cells• Following meiosis II, cytoplasm is lost and a tail

develops to allow locomotion• In some species this can occur year round while in other

species this is limited to certain times in the year.

Gamete Formation– Oogenesis – female gamete formation

• Occurs in the ovaries• Diploid germ cell (oogonium) undergoes meiosis to create 1

haploid cell• After meiosis I, cytoplasm does not split equally and the

majority goes to the primary oocyte. The other cell is called a polar body and it is not a viable sex cell.

• After meiosis II, the cytoplasm is again unequally divided and only one cell is viable as a sex cell (egg or ovum)

• In humans meiosis I begins in the ovarian tissue of the embryo before birth and does not continue beyond prophase I until puberty.

• Normally only one oogonium undergoes maturation each month

• Production of ova (many egg cell) continues from puberty until menopause (between 40 and 50).

Genetic Variation• Dependent on 2 factors:1. Crossing over – occurs

during prophase I and the number of which is determined by the chromosome size (usually 2 or 3 cross overs per chromosome).

Genetic Variation2. Random Segregation – how each pair of homologous chromosomes line up during metaphase I is also extremely important as that determines which pole the chromosomes will go to.

– These two factors work together and are the basis behind genetic recombination

– Page 171, fig 5.20

Errors in Meiosis• Nondisjunction – failure of chromosomes to separate properly

– Results in the addition or deletion of one or more chromosomes from a gamete

– If a gamete with an extra chromosome is fertilized by a normal gamete all the cells that develop from the zygote will also have an extra chromosome = trisomy• An example of this is Down’s Syndrome and this occurs when an

individual has an extra chromosome #21– If a diploid, rather then haploid, gamete unites with a normal

gamete the resulting zygote will have three sets of chromosomes (3n) and this is referred to as triploidy.

– Organisms that have more then 2 sets of chromosomes are called polyploids (rare in humans but common in plants such as seedless varieties of fruit)

Human Genetic Analysis• Karyotype – illustration or photograph

of the chromosomes in the nucleus of a somatic cell in an organism –46 chromosomes paired according to size,

shape and appearance.• Pedigree = used to determine if an allele

is dominant, recessive, autosomal or sex-linked.