CELLie’s Life

...a journey worth knowing

Heredity and Variation

Heredity and Variation

Heredity and Variation

Genetics

Genetics is the study of heredity and variation. It aims to understand how traits can be passed on to the next generation and how variation arises.

Heredity and Variation

Heredity - traits are passed on from parents to offspring.

Variation – demonstrates differences among individuals.

Example: physical similarities and differences

(eyes, nose skin, height)

abnormalities (color blindness, insanity, down syndrome, diabetes, cancer)

Cellular Reproduction

cells reproduce by dividing into two in the process called cell division

each dividing cell is called mother cell or parent cell, and its descendants are called daughter cells

the parent cell transmits copies of its hereditary information (DNA) to its daughter cells which in turn, pass it to their own daughter cells, becoming yet another parent cell, and so on and so forth

cell division is often referred to as cellular reproduction

most prokaryotic cells, by simply separating the

contents of the cell into two parts

eukaryotic cells can divide either through a process called mitosis or meiosis.

•  

CELL CYCLE

takes 24 hours for a mammalian cell to complete

involves the following events:

Periodic replication of DNA

Segregation of this replicated DNA with cellular

constituents to daughter cells

two general phases:

Interphase

Mitotic phase

Figure 1. Events in cell cycle

Mphase

INTERPHASE -consists of G1 phase, S phase, and G2 phase.

G1 PHASE

Gap1 phase

growth and increase in cell mass

preparation for DNA replication

lasts for about 10 hours complete

S phase

synthesis phase

DNA replication

protein synthesis

lasts for about nine hours

to

hours to complete

DNA REPLICATIONprocess of copying genetic material

results to two identical copies of DNA

transmits genetic information from cell to cell

during reproduction

Precise base-pairing during DNA replication:

Adenine (A) – Guanine(G)

Cytosine (C) – Thymine (T)

PROTEIN SYNTHESIS Process whereby DNA encodes for the

production of amino acids and proteins

can be divided into two parts:

Transcription

making a copy of part of the information in

DNA, thus forming the messenger RNA (mRNA)

Translation

converting that copied information, the mRNA,

into a protein with the aid of transfer RNA

(tRNA)

G2 PHASE

Gap2 phase

post DNA replication phase

shortest of the three phases

of

interphase

preparation for mitotic

cell division

lasts for about four hours

to complete Figure1

MITOTIC PHASEsignals the actual division of the cell

lasts for about one hour to complete

the two sister chromatids separate from each other,

one going to each of the two daughter cells

results to two daughter cells identical to each

other and to the parent cellFigure1

Two Types of Cell Division

Mitosis

Meiosis

Mitosis

comes from the Greek word mitos meaning “thread”

occurs in somatic or body cells

consists of four stages:ProphaseMetaphase AnaphaseTelophase

PROPHASE

phase of preparation

chromatin condense into chromosomes

nuclear membrane starts to breakdown

nucleoli become fragmented

and disperse in the cytoplasm

centrosomes move to opposite

poles and microtubules begin to form

METAPHASE

shortest phase

nuclear membrane

completely disappears

chromosomes assume

positions in the cell’s center

or equatorial plate

ANAPHASE

migration phase

centromeres separate

sister chromatids of each

chromosomes disengage and

move toward opposite

poles of the cell

each chromatid at the opposite

poles has its own centromere and is now

considered to be a single chromosome

TELOPHASE

phase of reconstruction

chromosome movement is completed

microtubules disassemble

nuclear membrane is reconstructed

around each daughter nucleus

nucleoli begin to reappear

chromosomes uncoil and

become more extendedFigure2

CYTOKINESIS

division of the cytoplasm

the surface around the equatorial

region of the cell pushes in toward

the center and pinches the cell

into two parts

two daughter cells are formed

Division

Figure 2. Mitosis in animal cell

Meiosis occurs only on reproductive cells or gametes

chromosomal material replicates once and cell divides twice

produce four daughter nuclei, each containing a haploid (n) number of chromosomes

has two successive divisionsreductive division or the Meiosis I equational division or the Meiosis II

Meiosis I

also known as reduction division

here the number of chromosome is reduced by one-half

homologous chromosomes pair, then segregate and move to different nuclei

consists of four stages:Prophase I

Metaphase I Anaphase I

Telophase I

Prophase I – chromosomes are duplicated and each consists of chromatids as in the process of mitosis

– but in here, homologous chromosomes pair with each other forming a tetrad of chromatids

– such pairing is called synapsis and does not occur in mitosis

Metaphase I – the pairs of homologous chromosomes line up across the spindle

Anaphase I – the homologous chromosomes separate from each other and move to the opposite poles of the cell

Telophase I – nuclear membrane forms around each group of chromosomes forming two genetically not identical daughter cells

– the separation of homologous chromosomes results in the segregation of genes that are on those chromosomes

Meiosis II

also known as equational division

the centromere splits and the sister chromatids separate into different nuclei

resembles a normal mitosisProphase IIMetaphase IIAnaphase IITelophase II

Differences Between Mitosis and Meiosis

MITOSIS occurs in somatic or body

cells direct cellular division

produce two diploid daughter cells

the daughter cells are exactly alike

MEIOSIS occurs in reproductive

cells or gametes has two successive

division (meiosis I and meiosis II)

produce four haploid daughter cells

the daughter cells are not all alike

LIFEYou don’t get to choose how your

going to die or when. You can only decide how you’re going to

live. - Joan

BuezSee into life, don’t just look at it.

- BaxterThe best use of life is to spend it for something that outlasts life.

- William James