economic english 10 pptx

Molecular Basis of Heredity

What an organism looks like and how it functions is

determined largely by its genetic material. The basic

principles of heredity were developed by Gregor Mendel,

who experimented with pea plants in the 19th century.

He mathematically analyzed the inherited traits (such as

color and size) of a large number of plants over many

generations. The units of heredity are genes carried on

chromosomes. Genetics can explain why children look

like their parents, and why they are, at the same time, not

identical to the parents.

Phenotype and Genotype

The collection of physical and behavioral characteristics

of an organism is called a phenotype. For example, your

eye color, foot size, and ear shape are components of

your phenotype. The genetic makeup of a cell or organ￾ism is called the genotype. The genotype is like a cook￾book for protein synthesis and use. Phenotype (what an

organism looks like or how it acts) is determined by the

genotype (its genes) and its environment. By environ￾ment, we don’t mean the Earth, but the environment

surrounding the cell or organism. For example, hor￾mones in the mother’s body can influence the gene

expression.

Reproduction

Asexual reproduction on the cellular level is called mito￾sis. It requires only one parent cell, which, after exactly

multiplying its genetic material, splits in two. The result￾ing cells are genetically identical to each other and are

clones of the original cell before it split.

Sexual reproduction requires two parents. Most cells

in an organism that reproduces sexually have two copies

of each chromosome, called homologous pairs—one

from each parent. These cells reproduce through mitosis.

Gamete cells (sperm and egg cells) are exceptions. They

carry only one copy of each chromosome, so that there

are only half as many chromosomes as in the other cells.

For example, human cells normally contain 46 chromo￾somes, but human sperm and egg cells have 23 chro￾mosomes. At fertilization, male and female gametes

(sperm and egg) come together to form a zygote, and the

number of chromosomes is restored by this union. The

genetic information of a zygote is a mixture of genetic

information from both parents. Gamete cells are manu￾factured through a process called meiosis, whereby a cell

multiples its genetic material once, but divides twice,

producing four new cells, each contains half the number

of chromosomes present in the original cell before divi￾sion. In humans, gametes are produced in testes and

ovaries. Meiosis causes genetic diversity within a species

by generating combinations of genes different from

those present in the parents.

–LIFE SCIENCE–

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Cytoplasm

Endoplasmic

reticulum

Plasma

membrane

Nucleolus

Nucleus

Vacuole

Cell wall

Ribosomes

Mitochondria

Centriole

Chloroplast

Lysosome

Plant Cell Animal Cell

Golgi

complex