Chapter 15 Cell Division

Homologous chromosomes

46 human chromosomes are 23 pairs of chromosomes. 23 chromosomes (mother )and 23 chromosomes ( father) 23 pairs of chromosomes are called homologous pairs (1) carry the same type of genetic information (eg. eye color, hair type, etc.) (2) are the same size and (3) have the same shape.

Meiosis: First division (Meiosis I) Anaphase I & Telophase I

Anaphase I The spindle fibers then pull the homologous chromosomes apart to opposite sides of the cell which starts to divide Telophase I Each side of the cell now has one of the homologous chromosomes. Every chromosome is made of two sister chromatids. A nuclear membrane starts to form. The cell splits into two .

A cell at a non-dividing stage?

Cell membrane Cytoplasm nuclear membrane Chromosomes are not clear in the nucleus

Development

Cell specialization follows cell division and allows an organism to develop and form organs adapted to perform a function

Name the structures that carry hereditary information

Chromosomes

Before cell division

Chromosomes are long, thin and not so visible Each chromosome is composed of two exact copies called chromatids, connected together at the centromere

Prophase

Chromosomes become shorter and thicker Nuclear membrane has disintegrated

Cytokinesis

Cytoplasm divides and identical daughter cells forming The 'original' parent cell no longer exists.

Independent Assortment

Different combinations of the chromosomes that lead to different gametes. This increases variation and explains why we are not identical to our parents or our brothers and sisters

Process of meiosis

Divided into First division and Second division

Somatic cells

Every species has a particular number of chromosomes in each cell. An onion cell: 4 chromosomes A human cell: 46 chromosomes A somatic cell is any body cell eg. eyes, skin, heart, liver etc. each of which has 46 chromosomes. Every somatic cell in your body contains an exact copy of the chromosomes present in the first cell from which your body grew (zygote).

Gametes in onion

Female gametes in the ovule of onion cells have 2 chromosomes. Male gametes in a plant cell are found in the pollen

Functions of mitosis

Growth Development Maintenance Asexual Reproduction

Gametes

Half the number of chromosomes Half the amount of inherited information. Gametes fuse to form the zygote in sexually reproducing organisms. Meiosis is a process involving reduction division forming such sex cells.

Sites of meiosis

In animal cells: Males - in the testes Females - in the ovaries In plant cells: Males - in the pollen sacs of the anthers Females - in the ovaries

Gametes in human

Male gametes are called sperms and are made in the testes. Sperm cells have 23 chromosomes (half the number of chromosomes in a somatic cell). Human female gametes are eggs or ova and are produced in the ovary.

Metaphase & Anaphase

Metaphase: Copied chromosomes line up at the center of the cell. Anaphasee: Sister chromatids separate and move to opposite ends of the cell. Spindle fibers assist in their movement The chromatids become the chromosomes of the two daughter cells

Two type of cell division

Mitosis and Meiosis

State four functions of mitosis

Mitosis functions for growth, maintenance, development, and asexual reproduction.

Mitosis

Mitosis is a nuclear division in which an exact copy of each chromosome is passed into two daughter cells. The daughter cells are identical to each other and identical to the original cell

Common sites of active mitosis In Plants

Not all cells divide , instead, mitosis is active in places called meristems 1. Root tip (Root meristem) 2. Shoot tip (Shoot meristem) 3. Cambium

Asexual reproduction

Offspring that result from asexual reproduction are just like the parent organism because they contain an exact copy of the same chromosomes ( vegetative probagation)

Which cells can divide?

Only cells with a nucleus can undergo cell division because the nucleus controls cell division

Functions of meiosis

Production of gametes in animals and plants

Meiosis: First division ( Meiosis I) Prophase I, Metaphase I

Prophase I Chromosomes become shorter and thicker and therefore become visible in the nucleus. The nuclear membrane disappears Metaphase I a. Homologous chromosomes lie alongside each other at the center of the cell b. Spindle fibers attach themselves to the centromere of each copied chromosome c. Crossing over events can take place at this phase. Genes on homologous chromosomes can be swapped. d. Independent assortment can take place at this phase too. The arrangement of chromosomes at the midline of the cell is random; how many paternal or maternal on either side is not under any control. This increases the number of possible chromosome combinations in the gametes produced

Meiosis: Second division (Meiosis II)

Prophase II Chromosome condense and become visible. The nuclear membrane starts to disintegrate Metaphase II Chromosomes line up at the center of the cell Spindle fibers form at the opposite ends of the cell aligning the sister chromatids in the middle of the cell Anaphase II The spindle fibers pull the two sister chromatids apart to the opposite ends of the cell which starts splitting Telophase II & cytokinesis Cell division is complete and a nuclear membrane forms around the chromatids (chromosomes of the new cell).

What is the chromosome number of the daughter cells resulting from meiosis called?

The chromosome number of the daughter cells resulting from meiosis is called haploid.

Haploid number of chromosome

The number of chromosomes in a gamete(n). In humans, sperm and the ova have the haploid number, n = 23

Diploid number of chromosome

The number of chromosomes present in a somatic cell (2n). In humans, the diploid number, 2n = 46

Replication

The process by which the chromosomes are copied. This occurs before the nucleus divides. Relication takes place during the interphase

Which condition concerning the cell structure should be observed so that cell division could take place?

The structural condition that should be observed so that cell division could take place is the presence of the nucleus.

Compare the daughter cells forming during mitosis

Two identical daughter cells result having the same number of chromosomes as the parent cell.

Structure of the chromosome

Two sister chromatids connected by a centromere

Maintenance

When a body is damaged (eg., following an injury), cell division will repair the damage and so maintain the body

Growth

When an organism grows, it generally produces more cells rather than form one big cell. This maintains a large surface area to volume ratio for diffusion of substances in and out of the cell to be at an optimum rate. All cells must have the same hereditary information as the cell from which they were formed, eg. growth of fetus from the embryo.

Meiosis-Comparison

a. Different genetic variations occur due to independent assortment b. Original chromosome number is halved, i.e., a haploid set is formed c. Results in production of haploid gametes d. Involved in sexual reproduction in plants and animals f. Gives rise to four daughter cells from one mother cell g. Occurs in reproductive organs (eg. anther, testis, ovaries) h. Two divisions i. Homologous chromosomes attach to the spindle in the first division j. Homologous chromosomes are separated in the first division. Sister chromatids are separated in the second division

Mitosis-Comparison

a. Exact duplication of genetic material b. Maintains the original chromosome number c. Results in diploid somatic cells d. Involved in growth, maintenance, development and e. Asexual reproduction in fungi and bacteria f. Gives rise to two daughter cells from one mother cell g. Occurs in meristematic regions of plants, growth regions of animals h. One division i. Chromatids attach randomly to the spindle j. Sister chromatids are separated

Common sites of active mitosis In Animals

a. Malpighian layer of the skin b. Bone marrow (production of blood cells) C. The lining of the intestines and other areas where cells are rapidly replaced

Chromosomes

are thread-like structures inside the nucleus These contain hereditary information:information inherited from the parent cells.

The halving of the number of chromosomes

is essential so that the total number of chromosomes remain constant from one generation to the next.

Meiosis

is often called reduction division because it halves the diploid number of chromosomes.

Telophase

nuclear membrane starts to form