HEREDITY IntroductionGenesProteinsChromosomesToo Many

Chromosomes

FertilizationMitosisMeiosisChromosomes in PairsRandom Assortment

INTRODUCTION

A gene is a stretch of DNA that specifies the sequence of a particular protien. In most organisms, hundreds or thousands of genes are arranged together in a specific order on enormously long pieces of DNA. These long DNA molecules (along with their associated proteins) are called chromosomes. Together all the chromosomes, collectively called the genome, carry the instructions for the construction, maintenance and growth of organisms.

Major features of a typical gene A gene is a segment of DNAthat specifies, or codes for, the sequence of a protein*. A gene also contains one or more regulatory sequences that either increase or decrease the rate of its transcription. Shown above is a picture of a long piece of DNA. The portion inside the blue rectangle represents a typical gene. Only the coding strand of the gene is shown. Various regions of the gene are indicated by different colored outlines:

Red: a region that encodes a protein sequenceBlack: a non-coding region (a single gene usually contains more than one) Green: a regulatory sequence

PROTEINSProteins are the most abundant kind of macromolecule found in the cells of living things. Proteins are composed of linear chains of subunits called amino acids. Human beings have tens of thousands of different kinds of proteins, and each one has a specific function. Proteins can be grouped into several general classes, including:

structural proteins, which provide physical support; e.g.collagen hormonal proteins, which regulate physiological processes; e.g. insulin respiratory proteins, which transport oxygen; e.g. myoglobin enzymes, which increase the rate of biochemical reactions; e.g.lysosome

CHROMOSOMESMany organisms bear more than one chromosome in their cells.

In fact, each species has a characteristic chromosome number. For example, Drosophilia has eight chromosomes in nearly all of its cells, tomatoes have 24, and humans have 46.

Each chromosome, in turn, has a characteristic size and shape, and each may stain with a distinctive pattern.

Each chromosome, in turn, bears a particular set of genes, and, as we'll see later, these genes are arranged in a specific order.

CHROMOSOMES

The fruit fly, Drosophila melanogaster, has eight chromosomes in the nucleus of almost all of its cells. Notice that the eight chromosomes are grouped into four pairs.

CHROMOSOMES

Human mitotic chromosomes numbered in order of size.

Too many chromosomes? In most organisms, chromosomes are passed from one generation to another as

a result of fertilization. At fertilization, the male and the female both contribute some chromosomes, and the new offspring thereby gets genes from both parents. That is why each of us inherits some characteristics from our mother and some from our father.

But this scheme has an inherent problem. Consider the case of humans. If the father and mother were each to contribute their entire set of 46 chromosomes, their children would have 92. When any of these children reproduced, their children would have a total of 184 chromosomes, 92 from each parent! If this were allowed to go on, the number of chromosomes in each generation would continue to double without end. Clearly, some mechanism must be in operation to keep the number of chromosomes consistent from generation to generation.

FERTILIZATIONFertilization is the

union of an egg and sperm. The fertilized egg is called a zygote.

This is a diagram of a human sperm. These cells are vehicles designed to carry DNA to the egg. As such they have a propulsion system (the tail), a source of energy, a nucleus, and little else. At the tip of the sperm is a specialized body called the acrosome, which helps the sperm penetrate the accessory coat of the egg.

FERTILIZATIONThis is a newly fertilized

sea urchin egg. It is about the same size as a mammalian egg. The raised membrane that surrounds the egg is called the fertilization membrane. It plays a role in ensuring that only one sperm fertilizes the egg.

MEIOSIS A mechanism to reduce chromosomes exists -- it is called meiosis and it occurs only in those

cells that are destined to become sex cells: sperm and eggs. In such cells, the number of chromosomes gets reduced by a factor of 2. Thus in Drosophila, instead of the eight chromosomes that are found in all other cells, the eggs and sperm carry only four chromosomes.

In the same way, the eggs and sperm of humans only carry 23 (rather than 46) chromosomes. So when a human sperm cell and a human egg unite, the sperm contributes 23 chromosomes to the 23 already present in the egg. This gives a total of 46, the diploid chromosome number.

All subsequent divisions occur via mitosis, and each cell that forms gets the full complement of 46 chromosomes.

The result is that all somatic (non-sex) cells get the sum of the chromosomes of the egg and sperm, yet the total number of chromosomes remains the same from one generation to the next.

MITOSIS SUMMARIZED

MITOSISThis somatic cell is

about to undergo mitosis.

MITOSISThe chromosomes

condense. For simplicity, only one pair of chromosomes are shown. The black chromosomes came from the organism's father and the red chromosomes came from its mother.

MITOSISThe nuclear membrane

dissolves and the chromosomes take up position on the spindle.

MITOSISThe chromosomes

move to opposite poles. The cell elongates in preparation for division.

MITOSISTwo cells are formed.

Each has a full complement of chromosomes. Soon the chromosomes will decondense.

Questions What exactly does it mean that a human father contributes 23

chromosomes to the next generation? Which ones? Are they always the same ones? If not, how does the father choose which ones are passed on?

What does it mean that a human mother contributes 23 chromosomes to the next generation?

Which ones? Are they always the same ones? If not, how does the mother choose which ones are passed on?

Pairs of Chromosomes Chromosome are distinctive: they carry a certain set of genes arranged in a

particular order. However, each chromosome is not unique: there are two copies of every chromosome in the somatic cells of most organisms.* The two chromosomes in each pair are said to be homologues. Each of the two homologous chromosomes has the same specific shape and staining pattern, and each carries the same characteristic set of genes in a specific order.

Since there are two copies of each chromosome in cells, it follows that there must be two copies of each gene in every somatic cell. (There are some well-known exceptions to this which will be discussed later.)

*The sex cells carry only one of each pair, and therefore half the number of chromosomes of somatic cells.

PAIRS OF CHROMOSOMES (HOMOLOGUES)

This picture shows 22 pairs of homologous chromosomes. Since this is the karyotype of a human male, the last two chromosomes are NOT homologous; they are the sex chromosomes X and Y.

A diagram of human chromosome 21. Both homologues are shown. The one from the mother is shown with blue bands and that from the father with red. Notice that the size, order of bands, and shape is the same for both chromosomes. The two chromosomes also carry the same genes in

the same order.

A photograph of the two homologues of human chromosome 2.Note their similar size, shape, and banding pattern.

MEIOSISMeiosis is preceded by

interphase. The chromosomes have not yet condensed.

MEIOSISThe chromosomes

have replicated, and the chromatin begins to condense.

MEIOSISThe chromosomes are

completely condensed. In meiosis (unlike

mitosis), the homologous chromosomes pair with one another

MEIOSISThe nuclear membrane

dissolves and the homologous chromosomes attach to the spindle fibers. They are preparing to go to opposite poles.

MEIOSISThe chromosomes

move to opposite ends of the cell.

MEIOSISThe cell begins to

divide into two daughter cells. It is important to understand that each daughter cell can get any combination of maternal and paternal chromosomes.

MEIOSISThe cell has divided

into two daughter cells.

MEIOSISAs in Meiosis I, the

chromosomes line up on the spindle fibers.

MEIOSISThe two cells each

begin to divide. As in Meiosis I, the chromosomes move to opposite ends of each cell.

MEIOSISWith the formation of

four cells, meiosis is over. Each of these prospective germ cells carries half the number of chromosomes of somatic cells.

Sex Cells

During the formation of the sex cells (gametes), organisms utilize the process of meiosis to pass on one of the two chromosomes in each pair to the next generation through their sperm or eggs.

But which one of each pair do parents "choose" to give to their offspring? The answer is that one or the other chromosome of each pair (one homologue) is chosen randomly.

Random Assortment The cell shown at the left is about to

enter the first meiotic division. For simplicity's sake, only two chromosomes are shown. The ones derived from the father are drawn in blue, while the maternal chromosomes are shown in red. There are two homologues of each chromosome; therefore, this cell is diploid. The cells on the next slide represent cells that could form as a result of the FIRST meiotic division. Any of them could eventually become a germ cell once meiosis is complete.

Notice that any combination of red or blue chromosomes may be passed on to the prospective germ cell as long as one of each homologue is respresented.

More Questions Why are there two of each kind of chromosome? What do you think is the advantage of having two copies of each chromosome and therefore

two copies of each gene? (Don't get confused by the fact that each chromosome consists of two strands of DNA -- one double stranded DNA molecule).

Are there any creatures with one of each kind of chromosome rather than two in their somatic cells?

Are there creatures with three or four or more of each kind of chromosome in their somatic cells?

If something goes wrong with meiosis, what might the consequences be for an organism if it got an uneven distribution of chromosomes: three of some kinds and one of some others ?

Can a species naturally have an odd number of chromosomes in its somatic cells? What might happen if two creatures with different chromosome numbers mated?