Chapter 9: Genes, chromosomes and patterns of inheritance 1

EL: To introduce genetics, with a focus on chromosomes

Before we begin..

• Answer questions 1-4 on page 291

Genetic instructions• Offspring receive genetic instructions from their

parents (or parent cells in the case of unicellular organisms)

• In humans, these instructions are packaged in gametes: the egg cells of a female and the sperm cells of a male

• We will be examining how these instructions come together to determine the characteristics of the offspring

Prokaryotic Chromosomes

Binary = two, Fission = splitting

• In prokaryotes, a single circular chromosome is attached to the plasma membrane at a specific point.

• When the cell divides by BINARY FISSION• DNA molecule replicates• The two copies are separated by the expansion of the plasma membrane• Plasma membrane and cell wall furrow inwards to divide the cytoplasm

resulting in two daughter cells.

DNA

• Deoxyribonucleic acid (DNA) is found within the nucleus of eukaryotic cells

• Chromatin is a mass of uncoiled DNA and associated proteins called histones.

• When cell division begins, DNA coils around the proteins forming visible structures called chromosomes.

Chromosome structure

Haploid cellsA cell with one set of chromosomes is called haploid (n) – gamete cells are usually haploid

Diploid cellsA cell with two sets of chromosomes is called diploid (2n) – somatic cells are diploid

Each matching pair is called “homologous” – they each contain the same genes – however, the DNA sequence isn’t necessarily the same

Polyploid cellsA cell with more than two sets of chromosomes is called polyploid. This is usually only found in plant cells.

Chromosome Numbers• An organism of a particular species always has the same number

of chromosomes (e.g. humans have 46 chromosomes or 23 pairs)• See table 9.1 page 292

Human chromosomes• Diploid number = 46, Haploid number = 23

• The 22 matched, homologous pairs of autosomal chromosomes are distinguished by:– Relative size– Position of centromeres– Patterns of light and dark bands when stained

• The 23rd pair in a diploid somatic cell are the sex chromosomes (N.B. In males these are NOT homologous)

Human KaryotypeKaryotype: the display of the number, size and shape of chromosomes from a cell

HUMAN FEMALE HUMAN MALEAutosomal chromosomes

Sex chromosomes

Activity

• In two groups, Complete Part A of activity 9.1 “Karyotypes” – one group will do figure 9.1C and the other group will do Figure 9.1D. Answer questions 1-5 i

• Re-visit Chapter 9 quick check questions 1-4 on page 291 (how did you go?) and complete qu 5-8 on page 301, question 5 on page 336

Reflection

These are the questions you should answer each lesson, preferably in writing•What learning was new today?•What learning was revision or built on what I already know?•What did I find most challenging and what strategies will I put in place to help me?•What percentage of the class did I spend on task and how can I improve this if needed?

Chapter 9: Genes, chromosomes and patterns of inheritance 2

EL: To introduce or revise mitotic cell division in eukaryotic somatic cells

What do you remember?

• Before we begin the lesson, write down or draw what you remember about eukaryotic cell division

Cell Division in Eukaryotes• A division of the

nucleus (mitosis) followed by a division of the cytoplasm (cytokinesis)

• To accomplish this task, the cell passes through a series of discrete stages or phases

http://www.youtube.com/watch?v=VlN7K1-9QB0

Interphase• Cells spend the majority of their

time (about 95%) in interphase.

• Cultured mammalian cells usually divide once every 18-24 hours.

• The cell appears “to be at rest”. Nothing could be further from the truth!

• During interphase most cellular contents are synthesised increasing cell mass. It is a time of cell growth, DNA replication and metabolic activity.

• The genetic material in the nucleus is in the form of chromatin fibres. Discrete chromosomes are not visible.

The G1 Phase• Interphase starts with G1 or

Growth 1; its the time for the cell to grow and carry out its biochemical activities. The length of this phase is highly variable between cells, typically 8-10 hours.

• Some cells sit in G1 for weeks, months, years. Cells that are arrested in G1 are said to be in a G0 state. Most nerve cells never leave G0.

• The decision to commit to cell division is made when the cell passes through the first checkpoint at the end of G1.

The S Phase• Once committed to cell

division the cell enters the S Phase – S stands for synthesis.

• This is the time for DNA replication. This typically takes 6-8 hours.

• The S phase ends when the DNA content of the cell has doubled. The evidence for this becomes obvious when the chromosomes become visible at the start of the M Phase. Each chromosome is now made up of two sister chromatids.

The G2 Phase• Once the genetic material has

doubled the cell now enters G2 – Growth 2. This phase is more fixed in its timing usually 4-6 hours for most cells.

• During this phase the cell actively prepares for cell division. It is a period of high metabolic activity and protein synthesis.

• The cell passes through checkpoint at the end of G2 to ensure that all is ready for the division of the nucleus.

• Needs lots of ATP.

The M Phase

• The M phase encompasses a division of the nucleus (mitosis) and then a division of the cytoplasm (cytokinesis).

• This phase explains how the two copies of the chromosomal DNA formed in S phase are separated and partitioned into daughter cells.

• The M phase lasts for less than 1 hour. The M phase is divided into various phases that are characterised by particular chromosome behaviour.

The M phase summary

Mitosis can be divided into five stages:

1.Interphase - cell performs all its normal functions. Before mitosis begins, DNA on replicates

2.Prophase – Nuclear membrane disappears

3. Metaphase - Spindle is visible and helps chromatids line up on equator

4. Anaphase - Chromatids get pulled to opposite poles.

5. Telophase - Two nuclei reform around the chromatids. The cell then divides (cytokinesis) into two daughter cells.

Activity

• Use your jelly snakes and jelly beans to model mitosis with a partner using the information on the upcoming slides

2. Prophase• Prophase beings when the individual

chromosomes have condensed to become discrete objects under the light microscope.

• In the cytoplasm, adjacent to the nucleus, the centrosomes, (duplicated in S phase) move to opposite ends of the cell. Spindle microtubules will form between these two centrosomes.

• Towards the end of prophase, the nuclear envelop breaks down

• The centrosomes are now at opposite ends of the cell and growing spindle microtubules enter the nuclear area and make contact with the chromosomes.

• Contact between a chromosome and spindle microtubules occurs at a protein – DNA complex region known as the kinetochore.

The relationship between the centromere, kinetochore and spindle microtubules.

3. Metaphase• Chromosomes are now

maximally condensed and lined up along the metaphase plate.

• Chromosomes can now be used in karyotype analysis.

• Metaphase occupies half the time required for mitosis.

• The chromosomes appear stationary, but each chromatid is being tugged towards the opposite poles by equally strong forces.

• In animal cells the centrosome contains a pair of centrioles.

4. Anaphase• The centromere holding the two

chromatids abruptly separates.

• Each chromatid (now a single chromosome) begins moving to opposite spindle poles as the microtubules get shorter and shorter.

• Anaphase is the shortest phase in mitosis typically lasting only a few minutes.

5. Telophase• Daughter chromosomes arrive

at the poles and revert to extended fibres of chromatin.

• The spindle microtubules disassembles and the nuclear membrane forms around the two groups of daughter chromosomes.

• During this period the cell usually undergoes cytokinesis – an independent process – that results in the division of the cytoplasm.

Cytokinesis

Animal Cell• Inward constriction of the plasma membrane results in cleavage furrow

during cytokinesis.

Plant Cell• Due to rigid cell wall, cytokinesis cannot constrict the plasma membrane

inwards. A new cell wall and plasma membrane is assembled across the cell plate.

• The result of mitosis and cytokinesis are two new daughter cells produced from one parent cell.

• The daughter cells contain the same (or virtually the same) genetic information and the same number of chromosomes as the parent cell.

Stage 5

Stage 4

Stage 1

Stage 2

Stage 3

http://www.youtube.com/watch?v=VlN7K1-9QB0

Checkpoints regulate the cell cycle• The cell cycle is highly regulated by intracellular signalling

molecules and extracellular signalling proteins

Defective Cell Cycle Control Mechanisms

• When control mechanisms fail, uncontrolled cell proliferation can produce a mass of cells called a tumour. Tumours can be benign or malignant (cancer).

• Mutations in the genes that express regulatory proteins accumulate. This leads to genetic instability and the development of cancer.

Animations and web links

• http://www.biology.arizona.edu/CELL_BIO/tutorials/cell_cycle/MitosisFlash.html

• http://www.johnkyrk.com/mitosis.html

Apoptosis• Apoptosis is programmed cell death or “cellular suicide”. It is a key

event in many biological processes. Removal of the tadpoles tail.

• The process is a specific sequence of events that result in the ordered dismantling of the internal contents of a cell.

• A key event is the activation of a series of enzymes called caspases.

• The pathway can be triggered by – (1) death signals or – (2) the withdrawal of survival factors.

• Mutations in genes that express proteins involved in apoptosis can lead to various cancers.

• NoBiology2 p.34-5

http://wehi.edu.au/education/wehitv/apoptosis_and_signal_transduction/

Activity

• Create a cell cycle poster with all the stages or mitosis mapped out

Reflection

These are the questions you should answer each lesson, preferably in writing•What learning was new today?•What learning was revision or built on what I already know?•What did I find most challenging and what strategies will I put in place to help me?•What percentage of the class did I spend on task and how can I improve this if needed?

Chapter 9: Genes, chromosomes and patterns of inheritance 3

EL: To introduce or revise meiotic cell division in eukaryotic gamete cells

Introduction to meiosis

• http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter12/animations.html#

Meiosis• The formation of gametes

(i.e.sex cells) - sperm and eggs- occurs by a special type of cell division called meiosis.

• The nuclei of sex cells contain only half as many chromosomes as the nuclei of all other cells (i.e. haploid) – called reduction division.

• When the nuclei of the sperm and egg join during fertilisation, the new cell then contains the full complement of chromosomes.

Meiosis• There are two divisions in meiosis; the first division is meiosis 1 and

the second is meiosis 2.

• The phases have the same names as those of mitosis. A number indicates the division number (1st or 2nd):– meiosis 1: prophase 1, metaphase 1, anaphase 1, and telophase 1– meiosis 2: prophase 2, metaphase 2, anaphase 2, and telophase 2

• In the first meiotic division, the number of cells is doubled but the number of chromosomes is not. This results in 1/2 as many chromosomes per cell.

• The second meiotic division is like mitosis; the number of chromosomes does not get reduced.

• See page 304 of your text book

Meiosis I

Meiosis 2

http://www.cellsalive.com/meiosis.htm

Activity

• Once again, use your jelly snakes and jelly beans to model mitosis with a partner using the information on the upcoming slides

Interphase

Interphase: Before meiosis begins, genetic material is duplicated. There are two homologous pairs of each chromosome (i.e. cell is diploid).

Meiosis 1: Prophase 1

• Duplicated chromatin condenses. Each chromosome consists of two, closely associated sister chromatids.

• Synapsis and crossing-

over occur during the latter part of this stage: two chromosomes of a homologous pair may exchange segments producing genetic variation.

Meiosis 1: Metaphase and Anaphase 1

• Metaphase 1: Homologous chromosomes align at the equatorial plate.

• Anaphase 1: Homologous pairs separate with sister chromatids remaining together.

Meiosis 1: Telophase 1

• Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair

• After Meiosis 1, there is usually a brief interphase

Meiosis 2

• Prophase 2: Spindle forms, DNA does not replicate.

• Metaphase 2: Chromosomes align at the equatorial plate.

Meiosis 2: • Anaphase 2:

Centromeres divide and sister chromatids migrate separately to each pole.

• Telophase 2: Cell division is complete. Four haploid daughter cells are obtained.

Animations

• http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter12/animations.html#

• http://www.cellsalive.com/meiosis.htm

Mitosis vs Meiosis

Activity

• Complete qu 1-11 of activity 9.2 on pages 90-91 of your activity manual (yes, you get to play with play doh!)

• Quick check questions 9-11 pg 306

• Make a poster of the stages of meiosis mapped out

Reflection

These are the questions you should answer each lesson, preferably in writing•What learning was new today?•What learning was revision or built on what I already know?•What did I find most challenging and what strategies will I put in place to help me?•What percentage of the class did I spend on task and how can I improve this if needed?

SAMPLE EXAM QUESTIONSTest revision

ANSWER = B

At the end of meiosis I females have two daughter cells and meiosis II only occurs if and when fertilization occurs by a sperm cell.

At that time both daughter cells divide to form 4 cells and of the 4 cells formed, 3 are discarded as polar bodies and the 4th cell having an enhanced cytoplasmic component combines its nuclear component with the sperm cell's nuclear component and crossing over occurs to form the embryo which then begins to divide via mitosis to become two cells, then four and so on.

An egg cell that is not fertilized is ovulated as a pair of daughter cells and there is no formation of polar bodies, hence, the eggs that are ultimately discarded at menstruation are not "finished" eggs. They have not undergone meiosis II.

ANSWER = C

ANSWER = C

ANSWER = A