The Cell Cycle

  • Uploaded by: Laxmikant Kamble
  • 0
  • 0
  • May 2020
  • PDF

This document was uploaded by user and they confirmed that they have the permission to share it. If you are author or own the copyright of this book, please report to us by using this DMCA report form. Report DMCA


Overview

Download & View The Cell Cycle as PDF for free.

More details

  • Words: 640
  • Pages: 48
The Cell Cycle

The Cell Cycle  A. The Role of Cell Division   Why do cells divide?   • Growth • Reproduction • Replacement of dying cells – skin, RBC • Reproduction in multi-cellular organisms – gamete formation (meiosis) In the case of growth, why divide, rather than simply get bigger? • Surface:volume ratio constraints

The Cell Cycle 1. 2. 3. 6. 7.

G1 Phase  1st growth phase S Phase  DNA duplicated G2 Phase  Final growth phase Mitosis Cytokinesis

Purpose of the first three phases (Interphase) – to duplicate cell contents; 90% of the cell’s growth cycle Purpose of Mitosis – to divide the genetic material into exact two halves Purpose of Cytokinesis – to divide all other contents (except nucleus) into two cells

Mitosis in an onion root

Binary fission in bacteria Not the best method of chromosome division – no microtubule and there is no division in exact halves of all genetic material. Bacteria have a single chromosome (versus the 46 human have).

Fig. 19.1

Important Notes DNA wraps some special proteins to form more stable structure called chromosomes Chromosome are found inside nucleus Human - 46 chromosomes, 23 pairs (1 set of 23 from egg, 1 set of 23 from sperm) Gene is a segment of DNA that is responsible for controlling a trait Each chromosome contains thousands of genes (these make up our traits)

Human female chromosomes

Human female karyotype

Human male chromosomes

Human male karyotype

Types of cell division • Mitosis: – Growth, development & repair – Asexual reproduction (yields identical cells) – Occurs in somatic (body) cells • Meiosis: – Sexual reproduction (yields different cells) – Occurs in specific reproductive cells

Process of mitosis   Aim: Two identical daughter cells exact copy of mother cell formed Requirements – the whole DNA must be copied exactly

Phases of Mitosis 1. Prophase 2. Prometaphase 3. Metaphase 4. Anaphase 5. Telophase

• Prophase -

Chromatin condenses, this causes the chromosomes to begin to become visible

-

Centrosomes separate, moving to opposite ends of the nucleus

-

The centrosomes start to form a framework used to separate the two sister chromatids called the mitotic spindle, that is made of microtubules

-

Nucleolus disappears

  2. Prometaphase - Nuclear envelope fragments - Chromosomes become more condensed - A kinetochore is formed at the centromere, the point where the sister chromatids are attached - Microtubules attach at the kinetochores

3. Metaphase - Chromosomes align on an axis called the metaphase plate - Note: the spindle consists of microtubules, one attached to each chromosome

1. Anaphase - Each centromere splits making two chromatids free - Each chromatid moves toward a pole - Cell begins to elongate, caused by microtubules not associated with the kinetochore

5. Telophase • Formation of nuclear membrane and nucleolus • Short and thick chromosomes begin to elongate to form long and thin chromatin • Formation of the cleavage furrow - a shallow groove in the cell near the old metaphase plate • Cytokinesis = division of the cytoplasm

Mitosis

Mitosis in an onion root

The human life cycle

Three sexual life cycles differing in the timing of meiosis and fertilization

Preparation of a human karyotype

Meiosis • Type of cell division that halves number of chromosomes (e.g., 2N to 1N) • Process of 2 successive divisions • Product is gamete, essential for sexual reproduction

Overview of meiosis: how meiosis reduces chromosome number

The results of alternative arrangements of two homologous chromosome pairs on the metaphase plate in meiosis I

The results of crossing over during meiosis

A comparison of mitosis and meiosis

A comparison of mitosis and meiosis: summary

Evolutionary advantage • asexual reproduction (mitosis) – easy, rapid, effective way to reproduce – useful in stable environment – lack of genetic diversity among offspring • sexual reproduction (meiosis) – promotes genetic variability – useful in dynamic environment

Related Documents

The Cell Cycle
December 2019 22
The Cell Cycle & Apoptosis
October 2019 33
The Cell Cycle
May 2020 13
Cell Cycle
July 2020 7
Cell Cycle
November 2019 33
Cell Cycle
November 2019 27

More Documents from ""