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How does a baby grow into an adult? Newborn baby has between 1 and 5 trillion cells Grown adult has between 50 and 100 trillion cells How does a wound heal? DNA Replication and Cell Division The importance of cells • All tissues in our bodies are made up of cells • Cells must be replicated: - to support growth (ex: baby to adult) - to replace damaged or dead cells (ex: healing a wound or mending broken bones) DNA – The instructions for life (Review – don’t copy) • The cell’s nucleus contains the master set of instructions that determines: - what the cell will become & how it will function • Instructions are carried in chromatin • Chromatin: long, thread-like pieces of DNA, divided into segments called genes DNA Replication • Before a cell divides, DNA replication occurs • The cell makes a copy of every chromatin strand • Copies are called sister chromatids DNA Replication Chromosomes: doubled chromatin strands wind up, shorten and thicken Chromatin vs. Chromosomes DNA replication DNA replicates Mitosis sister chromatids • identical strands centromere singlestranded chromatin May 5, 2017 centromere doublestranded chromatin doublestranded chromosome • Humans have 23 pairs of chromatin in each cell • All 23 pairs must be replicated before cell division can take place Determine all traits except sex Determine sex XX = female XY = male Centromere holds sister chromatids together Two copies: sister chromatids Cell Division • Once DNA replication is complete, cell division begins • Two steps: 1) mitosis (4 phase process) 2) cytokinesis 1. Mitosis • A four phase process in which the duplicated contents of a cell divide into two parts. 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase Your Task: a) Form groups of 4 students. b) Each member of the group is to become an expert on one phase of mitosis by reading the corresponding pages in your text (p. 34 – 35) and filling in the handout. c) Teach the other students in your group about the phase you studied and complete the handout together. d) If time permits: Take a set of images of mitosis from the front counter and try to put them in the correct order in your group. Homework • Read pages 37-38 (on cytokinesis) • Mitosis Cut & Paste Review • http://iknow.net/player_window.html?url=m edia/prophase_video_auto.swf&width=360 &height=285 CELL CYCLE – PROPHASE 1.Prophase centrioles spindle fibres doublestranded chromosomes Stages of Mitosis centrioles move to the opposite ends of the cell spindle fibers form between centrioles chromatin coils up into chromosomes nuclear membrane & nucleolus disintegrate chromosomes hook onto spindle fibers by their centromeres May 5, 2017 CELL CYCLE – METAPHASE 2. Metaphase centrioles Stages of Mitosis spindle fibres shorten or lengthen pulling chromosomes to align in the middle (equator) of the cell sister chromatids spindle fibres May 5, 2017 CELL CYCLE – ANAPHASE 3.Anaphase Stages of Mitosis centromeres split & each half pulls the chromatids to opposite ends chromosomes are now singlestranded single-stranded chromosomes May 5, 2017 CELL CYCLE – TELOPHASE 4.Telophase Stages of Mitosis 2 new identical spindle fibres are disassembled nuclei centrioles replicate nucleoli & nuclear membrane reappear chromosomes unwind into chromatin cytokinesis begins cytokinesis May 5, 2017 2. Cytokinesis • The division of the cell into two daughter cells • Begins near the end of telophase Cytokinesis in animal cells • Ring of specialized proteins at middle of cell start to contract • Pinches cell membrane until parent cell becomes two daughter cells • Each daughter cell has cytoplasm, organelles & complete set of chromosomes in a nucleus Contraction of proteins and pinching of membrane Two daughter cells produced Cytokinesis in plant cells • Golgi body produces small vesicles that carry materials to make new cell wall • Vesicles line up at centre of cell forming cell plate • Cell plate grows out to meet old cell wall new wall formed separating two daughter cells Comparing cytokinesis in animal and plant cells Homework • Stages of Cell Cycle Worksheet